LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


GIFT    OF 


.PROF.  W.JB.  RISING 

Class 


STANDARD  WORKS  ON  CHEMISTRY. 


WOHLER'S  ORGANIC  CHEMISTRY— Lately  Issued. 
OUTLINES   OF   ORGANIC    CHEMISTRY.      By  WOHLER 

and  FITTIG.  Translated,  with  Additions,  from  the  Eighth  German 
Edition,  by  IRA  RKMSEN,  M.D.,  Ph.D.,  Professor  of  Chemistry  in 
Johns  Hopkins  University,  Baltimore.  In  one  handsome  volume, 
12mo.,  of  550  pp.  ;  cloth,  $3. 

As  a  book  of  general  reference,  we  do  not  hesitate  to  recommend  it  as  the  best  and 
most  satisfactory  we  have  seen,  and.  in  fact,  it  seems  to  have  left  nothing  to  be  de- 
sired from  such  a  publication. — Journal  of  Applied  Chemistry,  Sept.  1873 

This  compact  treatise  is  an  excellent  handbook  of  organic  chemistry  for  the  student 
attending  chemistry  or  engaged  in  laboratory  work.  The  notation  is  given  in  the 
simplest  graphic  form,  and  the  relations  of  the  various  classes  of  bodies  and  of  indi- 
vidual substances  to  each  other  are  easily  traced.  Dr.  Remsen  has  discharged  his 
duty  as  translator  in  a  thoroughly  acceptable  manner. — Silliman's  Journal,  July,  1873. 

FOWNES'  ELEMENTARY  CHEMISTRY— Lately  Issued. 

A  MANUAL  OF  ELEMENTARY  CHEMISTRY,  THEO- 
RETICAL AND  PRACTICAL.  By  GEORGE  FOWNES,  Ph.D  ,  late 
Professor  of  Practical  Chemistry  in  University  College,  London.  A 
new  American,  from  the  Tenth  .and  Revised  London  Edition  Edited 
by  ROBERT  BRIDGES,  M.D.  In  one  large  royal  12rao.  volume  of 
about  850  pp.,  with  197  illustrations  ;  cloth,  $2  75  ;  leather,  $3  25. 

BOWMAN'S  MEDICAL  CHEMISTRY— Lately  Issued. 
A  PRACTICAL  HANDBOOK  OF  MEDICAL  CHEMIS- 
TRY. By  JOHN  E.  BOWMAN,  M.D.  Edited  by  C.  L.  BLOXAM,  Pro- 
fessor of  Practical  Chemistry  in  King's  College,  London.  Sixth 
American,  from  the  fourth  and  Revised  English  Edition.  In  one 
neat  volume,  royal  12mo.,  pp.  351,  with  numerous  illustrations ; 
cloth,  $2  25. 

BOWMAN'S  MEDICAL  CHEMISTRY— Lately  Issued. 
AN   INTRODUCTION   TO    PRACTICAL    CHEMISTRY, 
INCLUDING  ANALYSIS.      By  JOHN  BOWMAN,  M.D.    Sixth  Ame- 
rican, from  the  si^th  and  Revised  London  Edition.    With  numerous 
illustrations.     In  one  neat  volume,  royal  12mo. ;  cloth,  $2  25. 

ODLING'S  CHEMISTRY— Lately  Issued 

A  COURSE  OF  PRACTICAL  CHEMISTRY,  arranged  for 
the  use  of  Medical  Students.  By  WILLIAM  ODLING,  Lecturer  on 
Chemistry  at  St.  Bartholomew's  Hospital,  <fce.  With  illustrations. 
From  the  Fourth  and  Revised  London  Edition  In  one  neat  royal 
12mo.  volume;  cloth,  $2. 

RODWELL'S  DICTIONARY  OF  SCIENCE— Lately  Issued. 
A  DICTIONARY  OF  SCIENCE:  comprising  Astronomy, 
Chemistry,  Dynamics  Electricity,  Heat,  Hydrodynamics,  Hydrosta- 
tics, Light,  Magnetism,  Mechanics,  Meteorology,  Pneumatics,  Sound, 
and  Statics.  Preceded  by  an  Essay  on  the  History  of  the  Physical 
Sciences.  By  G.  F.  RODWKLL,  F.R  A.S.,  &c  In  one  handsome  oc- 
tavo volume  of  694  pp.,  and  many  illustrations;  cloth,  $5. 


HENRY  C.  LEA,  Philadelphia. 


ATTFIELD'S  CHEMISTRY,  NEW  EDITION— Now  Ready. 
CHEMISTRY,  GENERAL,  MEDICAL,  AND  PHARMA- 
CEUTICAL, INCLUDING  THE  ^CHEMISTRY  OF  THE  U.  S. 
PHARMACOPOEIA.  A  manual  of  the  General  Principles  of  the 
Science,  and  their  applications  in  Medicine  and  Pharmacy.  By 
JOHN  ATTFIELD,  Ph.D.,  F  C.S,,  Professor  of  Practical  Chemistry  to 
the  Pharmaceutical  Society  of  Great  Britain.  Seventh  American, 
revised  from  the  Sixth  English  Edition  by  the  author.  In  one  very 
handsome  12rno.  volume  of  668  pages,  with  87  illustrations  :  cloth, 
$2.75;  leather,  $3.25. 

The  enviable  reputation  already  gained  by  this  standard  work  will  certainly  be 
mucb  increased  b.y  the  many  improvements  incorporated  in  the  new  edition,  which 
has  just  appeared.  A  number  of.  valuable,  well  executed,  illustrations  of  chemical 
apparatus  have  been  added,  the  recent  changes  in  the  nomenclature  of  the  U.  S. 
Pharmacopoeia  are  duly  noted,  and  the  whole  work  revised  so  as  to  be  abreast  with 
the  many  late  discoveries  in  chemical  science  At  the  same  time  the  thoroughly 
practical  character  of  the  previous  editions  is  still  maintained. — Chicago  Med.  Journ. 
and  Examiner,  Jan.  1877. 

After  having  used  it  as  a  text-book  in  the  laboratory  of  the  Philadelphia  College  of 
Pharmacy  during  the  last  five  years,  wepan  speak  from  our  own  experience  and  testify 
to  its  intrinsic  value  in  the  instruction"^  the  student.  The  more  we  have  used  it, 
the  more  we  were  pleased  with  it,  ancUon-^he  appearance  of  a  new,  revised,  and  en- 
larged edition,  we  take  occasion  to  again  cordially  recommend  it,  believing  that  for 
the  practical  instruction  of  pharmaceutical  students  in  chemistry  it  has  no  superior 
in  the  English  language. — Am.  Journ.  of  Pharm.,  Nov.  1876. 


BLOXAM'S  CHEMISTRY— Lately  Issued. 

CHEMISTRY,  INORGANIC  AND  ORGANIC.  By  C  L. 
BLOXAM,  Professor  of  Chemistry  in  King's  College,  London.  From 
the  Second  London  Edition.  In  one  very  handsome  octavo  volume 
of  700  pages,  with  about  300  illustrations;  cloth,  $4  ;  leather,  $5. 

The  above  is  the  title  of  a  work  which  we  can  most  conscientiously  recommend  to 
students  of  chemistry.  It  is  as  easy  as  a  work  on  chj^iwrtry  could  be  made,  at  the 
same  time  that  it  presents  a  full  account  of  that  science  asii  now  stands.  We  have 
spoken  of  the  work  as  admirably  adapted  to  the  wants  of  sludents  ;  it  is  just  as  well 
suited  to  the  wants  of  practitioners  who  wish  to  review  their  chemistry,  or  have  oc- 
casion to  refresh  their  memories  on  any  point  relating  to  it.  In  a  word,  it  is  a  book 
to  be  read  by  all  who  wish  to  know  what  is  the  chemistry  of  the  present  day. — Ameri- 
can Practitioner,  Nov.  1873. 

We  cordially  welcome  this  American  reprint  of  a  work  which  has  already  won  for 
itself  so  substantial  a  reputation  in  England.  Professor  Bloxam  has  condensed  into 
a  wonderfully  small  compass  all  the  important  principles  and  facts  of  chemical  science. 
Thoroughly  embued  with  an  enthusiastic  love  for  the  science  he  expounds,  he  has 
stripped  it  of  all  needless  technicalities,  and  rounded  out  its  hard  outlines  by  a  ful- 
ness of  illustration  that  cannot  fail  to  attract  and  delight  the  student.  The  details  of 
illustrative  experiment  have  been  worked  up  with  especial  care,  and  many  of  the 
experiments  described  are  both  new  and  striking. — Detroit  Rev.  of  Med.  and  Pharm., 
Nov.  1873.  

CLOWES'  CHEMISTRY— Nearly  Ready. 

AN  ELEMENTARY  TREATISE  ON  PRACTICAL  CHEM- 
ISTRY AND  QUALITATIVE  INORGANIC  ANALYSIS.  Espe- 
cially Adapted  for  Students  in  Laboratories  of  Schools  and  Colleges 
and  by  Beginners.  By  FRANK  CLOWES,  D.Sc.,  London.  From  the 
Second  Revised  English  Edition,  with  about  fifty  illustrations  on 
wood.  In  one  very  handsome  royal  12mo  volume. 

The  arrangement  in  tabular  form  of  all  special  analytical  processes  has  enabled 
the  author  to  compress  a  very  large  amount  of  instruction  within  a  reasonable  com- 
pass, and  in  a  form  exceedingly  clear  and  intelligible,  rendering  the  volume  one  es- 
pecially suited  for  practical  work  in  the  laboratory,  or  for  the  guidance  of  students 
deprived  of  the  assistance  of  a  teacher. 

HENRY  C.  LEA,  Philadelphia. 


PRINCIPLES 


THEORETICAL 


CHEMISTRY, 


WITH  SPECIAL  REFERENCE  TO  THE 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS. 


BY 


IRA  REMSEN,  M.D.,PH.D., 

PROFESSOR    OP    CHEMISTRY    IN    THE    JOHNS    HOPKINS    UNIVERSITY. 


PHILADELPHIA: 

HENRY     C.     LEA 

1877. 


Entered  according  to  the  Act  of  Congress,  in  the  year  1877,  by 

HENRY     C.    LEA, 
in  the  Office  of  the  Librarian  of  Congress.     All  rights  reserved. 


PHILADELPHIA: 

COLLINS,   PRINTER, 

70.5  .Tayue  Street. 


PREFACE. 


THIS  little  book  is  intended  to  furnish  the  student  with 
a  simple  statement  of  the  fundamental  principles  of  what 
is  commonly  called  Theoretical  Chemistry.  The  subject 
is,  of  course,  not  exhausted  ;  many  things  have  purposely 
been  left  out,  either  because  they  have  not  yet  reached 
such  a  stage  of  development  as  to  entitle  them  to  a  place 
among  the  fundamental  principles,  or  because  it  was 
thought  better  to  emphasize  more  strongly  those  princi- 
ples which  are  treated.  Should  the  reader  miss  anything 
which  he  expected  to  find,  he  will  please  carefully  con- 
sider whether  the  grounds  referred  to  are  a  sufficient 
excuse  for  the  omission. 

The  imperfections  that  will  be  noticed  are,  partly  at 
least,  due  to  the  imperfection  of  our  knowledge  on  some 
of  the  subjects  discussed.  For  instance,  it  seems  to  be 
impossible  for  us  at  present  to  treat  the  subject  of  Valence 
in  such  a  way  as  to  lead  to  satisfactory  results,  mainly  for 
the  reason  that  we  know  so  little  in  regard  to  it.  What- 
ever view  of  this  property  one  may  take,  he  will  find  some 
difficulties  which  he  cannot  surmount. 

As  for  the  value  of  the  structural  formulas,  which  are 
discussed  at  some  length  in  the  second  part  of  the  book, 
it  need  only  be  said  that,  if  it  be  borne  in  mind  what  they 
are  intended  to  represent,  they  are  not  quite  so  absurd  as 
some  chemists  are  just  now  trying  to  make  us  believe 
the}^  are.  These  formulas  certainly  represent  known 
facts  in  regard  to  the  constitution  of  chemical  com- 

1* 

237460 


VI  PREFACE. 

pounds.  They  do  not  represent  these  compounds  as  a 
photograph,  for  example,  represents  a  building;  but 
rather  somewhat  in  the  same  way  that,  in  Physics,  lines 
represent  forces  in  their  magnitude  and  direction.  Take 
the  formulas  for  what  they  are,  and  they  have  consider- 
able value.  Try  to  find  in  them  the  architectural  plans 
of  the  chemical  molecules,  and  they  appear  absurd.  But 
it  is  very  unjust  to  find  fault  with  a  thing  for  not  doing 
what  it  never  pretended  to  do,  and  what  its  originators 
have  distinctly  stated  it  could  not  do. 

A  careful  study  of  this  book  will,  it  is  believed,  be  of 
assistance  in  showing  exactly  upon  what  basis  our  con- 
ceptions of  chemical  constitution  rest.  Whether  our 
ideas  are  good  or  bad,  they  deserve  to  be  studied,  for 
the  simple  reason  that  they  are  held  by  nearly  all  the 
working  chemists  of  the  da}',  and  much  of  the  work  that 
is  being  done  in  the  principal  laboratories  is  a  result  of 
these  prevailing  ideas. 

I.  R. 

BALTIMORE,  February,  1877. 


TABLE   OF   CONTENTS. 


PART  FIRST:    GENERAL  DISCUSSION  OF 
ATOMS  AND  MOLECULES. 

PAGE 

I.  ATOMIC  THEORY — ATOMIC  WEIGHTS,  ETC.         .         .         .13 

General  conceptions — Chemism — Dalton's  investiga- 
tions— Atomic  theory — Determination  of  atomic  weights 
— Methods  for  the  determination  of  atomic  weights  de- 
pendent upon  analysis — Equivalents — Determinations  by 
Berzelius — The  principle  of  substitution  employed  in  the 
determination  of  atomic  weights — Consideration  of  che- 
mical decompositions  for  the  purpose  of  determining 
atomic  weights  —  Elements  —  Compounds  —  Mechanical 
mixtures— Solutions  and  alloys. 

II.  EXAMINATION  OF  GASEOUS  ELEMENTS  AND  COMPOUNDS   .       32 

Investigations  of  Gay  Lussac — Avogadro's  speculations 
— Determination  of  molecular  weights — Number  of  atoms 
in  the  molecules  of  elements— Molecules  of  elements  con- 
taining more  or  less  than  two  atoms — Varying  number  of 
atoms  in  the  molecule  of  one  and  the  same  element — 
Other  proofs  of  the  fact  that  the  molecules  of  elements 
contain  more  than  one  atom — Molecular  formulas  of 
gaseous  compounds— Apparent  exceptions. 

III.  EXAMINATION  OF  SOLID  ELEMENTS  AND  COMPOUNDS       .       56 
Specific  heat— Relations   between    specific  heat   and 

atomic  weights — Investigations  of  Dulong  and  Petit — 
Investigations  of  Neumann  and  Regnault — Determina- 
tion of  atomic  weights  by' a  study  of  the  specific  heat  of 
compounds — Exceptions  to  the  law  of  Dulong  and  Petit. 


.    Isomorphism  as  furnishing  a  Means  for  determin- 
ing Atomic  Weights 68 

IV.  PROPERTIES  OF  THE  ELEMENTS  AS  FUNCTIONS  OF  THEIR 

ATOMIC  WEIGHTS      .         .         .         .         .         .         .70 

Natural  groups  of  elements— The  scheme  of  Mendelejeff 
— Lothar  Meyer's  arrangement  of  the  elements. 


Vlii  CONTENTS. 

PAGE 

V.  VALENCE  OR  ATOMICITY  OF  ELEMENTS     .         .         .         .79 

Definition — Name  of  the  new  property — Distinction 
between  valence  and  affinity — Methods  for  determining 
the  valence  of  the  elements — Insufficiency  of  the  hypothesis 
— Atomic  and  molecular  compounds— Foundation  for  the 
distinction  between  atomic  and  molecular  compounds- 
Use  of  the  distinction — Difficulties  met  with — Experi- 
ments showing  that  nitrogen  may  be  both  trivalent  and 
quinquivalent — The  distinction  between  atomic  and  mole- 
cular compounds  unnecessary  as  far  as  the  hypothesis  of 
valence  is  concerned — Saturated  and  unsaturated  com- 
pounds— Double  union — Variable  valence — Objections  to 
the  idea  of  variable  valence — Wiirtz's  view — True  valence 
— Methods  for  determining  true  valence — Apparent  va- 
lence—  Conclusions. 


PART  SECOND:    CONSTITUTION  OR  STRUC- 
TURE OF  CHEMICAL  COMPOUNDS. 

I.  GENERAL  CONSIDERATIONS— DEFINITION  OF  CONSTITUTION, 

ETC.    .  . -.  .99 

Definition — Possible  forms  of  combination— Types — 
Residues— Chains — Double  and  treble  union. 

Classes  of  Compounds       ......     Ill 

Acids— Hydrogen  acids — Hydroxyl  acids — Proofs — 
Further  experiments  necessary  in  most  cases  —  Other 
acids — Subdivision  of  acids — Bases — Differences  between 
acids  and  bases— Complex  bases— Salts — Complex  Salts 
— Anhydrides — Proofs  of  the  constitution  of  anhydrides 
— Oxides — Analogy  between  salts  and  anhydrides  and 
oxides. 

Compounds  of  Carbon 124 

Hydrocarbons  —  Homologous  series  —  Experimental 
proofs  —  Alcohols  —  Proofs  —  Classes  of  alcohols  —  Pri- 
mary alcohols— Secondary  alcohols — Proofs  of  the  general 
formula  of  secondary  alcohols — Tertiary  alcohols — Proofs 
—  Determination  of  alcohols  —  Mercaptans  —  Acids  — 
Proofs— Methods  for  the  formation  of  the  acids  of  carbon 
— Aldehydes  —  Proofs  —  Acetones  —  Proofs  —  Ethers — 
Proofs— Simple  ethers — Proofs—  Anhydrides — Peculiar 
anhydrides. 


CONTENTS.  IX 

PAGE 

Substitution  Products  .  .  .  .  .'.  .  148 
Substitution  products  containing  chlorine,  bromine,  or 
iodine  — Complex  substitution  products — Constitution  of 
substituting1  groups — Constitution  of  the  group  CN — 
Constitution  of  the  group  SO,H  — Constitution  of  the 
group  N0.2 — Constitution  of  the  group  NO— Constitution 
of  the  group  NH2 — Constitution  of  the  group  NH. 

II.    SPECIAL   STUDY   OF   THE   CONSTITUTION   OF   CHEMICAL 

COMPOUNDS        .        .         .        .       '•'. '       '.  . '    -.         .     162 
Isomerism. 

Compounds  not  containing  Carbon  ....  163 
Compounds  of  chlorine,  etc.,  with  oxygen,  and  oxygen 
and  hydrogen— Compounds  of  sulphur,  etc.,  with  oxygen, 
and  oxygen  and  hydrogen— Sulphurous  acid,  HJSOa — 
Sulphuric  acid,  H.2SO4 — Pyrosulphuric  acid,  H.,S.207 — 
Hyposulphurous  or  thiosulphuric  acid,  H2S2O3 — Other 
poly th ionic  acids — Compounds  of  nitrogen  with  oxygen, 
and  with  oxygen  and  hydrogen — Nitrous  acid,  HNO2 — 
Nitric  acid,  HNOfl — Hydroxylamine,  H3NO— Compounds 
of  phosphorus  with  oxygen,  and  with  oxygen  and  hydrogen 
— Hypophosphorous  acid,  HSP02 —  Phosphorous  acid, 
H3P03 — Phosphoric  acid,  H3P04 — Pyrophosphoric  acid, 
H1P.2O7 — Metaphosphoric  acid,  HP03— Compounds  of 
boron  with  oxygen,  and  with  oxygen  and  hydrogen — 
Compounds  of  silicon  with  oxygen,  and  with  oxygen  and 
hydrogen — Salts — Ammonium  salts  — Salts  of  copper  and 
mercury— Salts  of  iron  and  chromium— Salts  of  aluminium 
— Metal  acids— Compounds  of  uranium. 

Constitution  of  Carbon  Compounds  ....     182 
Methane  Derivatives.     (Fatty  Bodies.) 

First    Group :     Bodies  derived    from    the    hydrocarbons 

CnH2n+2 182 

Derivatives  of  ethane,  C2H6 — Derivatives  of  propane,  C3H8 
— Derivatives  of  butane,  C4H10 — Proofs — Derivatives  of 
pentaue,  C5H12— Proofs— Derivatives  of  hexane,  C6HU^— 
Proofs— Derivatives  of  heptane,  CTH]B — Monobasic,  mon- 
atomic  acids,  CnH2n02 — Propionic  acid — Butyric  acids — 
Proofs — Valeric  acids — Proofs — Caproic  acids — Proofs 
— Aldehydes — Acetones  or  ketones. 

Second   Group :    Bodies  derived  from   the   hydrocarbons 

CBH2n        ....  ....     196 

Ethylene  and  derivatives— Propylcne,  etc. — Alcohols 
—Proofs  —  Acids. 


CONTENTS. 


PAOE 


Third  Group:     Bodies    derived   from    the   hydrocarbons 

CnH2n_2    .         .        . 199 

Fourth  Group  :  Diatomic  alcohols  and  acids      .        .        .     200 

Diatomic     alcohols,     CnH2n+202  —  Diatomic     acids, 
CnH2n03— Proofs— Bibasic  acids,  CnH2n_204— Proofs. 

Fifth  Group  :  Triatomic  alcohols  and  acids        .        .        .     205 
Glycerin. 

Sixth  Group :  Tetratoraic  compounds        .        .        .        .    206 

Seventh  Group  :  Cyanogen  compounds      ....     207 
Mustard  oils. 

Eighth  Group  :  Derivatives  of  carbonic  acid     .         .         .     209 
Benzene  Derivatives.     (Aromatic  Bodies.)        .         .     211 
Constitution  of  benzene— Substitution  products  of  ben- 
zene—Bi-substitution  products— Tri-substitution  products 

— Peculiar  benzene  derivatives — Phenoles— Quinones 

Azo-  and  diazo-bodies. 

Naphthalene  Derivatives 225 

Anthracene  Derivatives     .         •  226 


LIST  OF   ELEMENTS  AND  ATOMIC  WEIGHTS. 


Name  of  Element. 

Symbol. 

Atomic 
weight. 

Atomic  weight, 
very  accurately 
determined. 

Aluminum        fc        .    "  •  . 

Al 

27.4 

Antimony          . 

Sb 

122 

Arsenic     . 

As 

75 

Barium      .         .        ... 

Ba 

137 

Bismuth    ...         .        » 

Bi 

210 

Boron      '-.        .        . 

B 

11 

Bromine    .        .        ,  •      . 

Br 

80 

79.952 

Cadmium  ...        .        .        . 

Cd 

112 

Caesium     .        .      ;  »   -<•    .  •        . 

Cs 

133 

Calcium    .        .        ... 

Ca 

40 

Carbon 

C 

12 

Cerium      .         .         ... 

Ce 

92 

Chlorine    .        .         .        .    '     . 

Cl 

35.5 

35.457 

Chromium         .  •      .        . 

Cr 

52.4 

Cobalt       .        .        .        .        . 

Co 

59 

Columbian!       .        .        . 

Cb 

94 

Cu 

63  4 

. 

Didymium         .         . 

D 

9<> 

E 

113  7 

Fluorine    ..... 

F 

19 

Gallium     ..... 

Ga 

__ 

Gl 

9.4 

Gold           

An 

197 

Hydrogen          .        . 

H 

1 

Indium      ..... 

In 

113.7 

Iodine        ..... 

I 

127 

126.85 

Iridium     ..... 

Ir 

198 

Iron  .        . 

Fe 

56 

Lanthanum       . 

La 

92.5 

Pb 

207 

Lithium     .        .        r        .        . 

Li 

7 

7.022 

Magnesium        .         .                  . 
Manganese    •    .         . 

Mg 
Mn 

24 
55 

Mercury    .        .         .        .         .. 

Hg 

200 

Molybdenum     .... 

Mo 

95.8 

Nickel       

Ni 

59 

Nitrogen   ..... 

N 

14 

14.044 

Xli        LIST    OF    ELEMENTS    AND    ATOMIC    WEIGHTS. 
LIST  OP  ELEMENTS  AND  ATOMIC  WEIGHTS. —  Continued. 


Name  of  Element. 

Symbol. 

Atomic 
weight. 

Atomic  weight, 
very  accnraiely 
determined 

Osmium     .         .              •    .        , 

Os 

199.2 

Oxygen     .         .         ..... 

0 

16 

Palladium          ... 

Pd 

106.6 

Phosphorus       .         .  '      . 
Platinum  ..... 

P 
Pt 

31 
197.4 

Potassium          .... 

K 

39.1 

39.137 

Rhodium  .        .        . 

Ro 

104.4 

Rubidium          .         .         . 

Rb 

85.4 

Ruthenium        .         ... 

Ru 

104.4 

Selenium  .        ... 

Se 

79.4 

Silicon       .         .         ... 

Si 

28 

Silver        .....     .  ,     . 

Ag 

108 

107.93 

Sodium     ..... 

Na 

23 

23.043 

Strontium          .... 

Sr 

87.5 

Sulphur     .         . 

S 

32 

Tantalum           .... 

Ta 

182 

Tellurium          .... 

Te 

128 

Thallium  

Tl 

204 

Thorium   

Th 

231 

Tin             

Sn 

118 

~ 

Titanium  .        . 

Ti 

50 

Tungsten  . 
Uranium   .         .  .    ,.         . 

W 
U 

184 
240' 

Vanadium 

V 

51.3 

Yttrium    .        . 

Y 

61.7 

Zinc  .         ... 

Zn 

65 

Zirconium         ...       .                 . 

Zr 

89.6 

PRINCIPLES 

OP 

THEORETICAL  CHEMISTRY. 


PART  FIRST. 

GENERAL  DISCUSSION  OF  ATOMS 
AND  MOLECULES. 


I. 

ATOMIC  THEORY— ATOMIC  WEIGHTS,  ETC. 

General  Conceptions. — Substances  that  occur  in  nature 
are,  for  the  most  part,  not  simple  substances.  They  can 
generally  be  decomposed  into  kinds  of  matter  that  are  un- 
like— some  by  one  means,  some  by  another.  Such  sub- 
stances are  called  compound.  In  regard  to  the  means 
required  to  separate  these  compounds  into  their  constitu- 
ents, a  marked  difference  is  noticed  in  different  cases. 
Sometimes  only  very  simple  mechanical  processes  are 
required  to  effect  the  separation.  At  other  times  it  is 
found  that,  after  the  application  of  all  purely  mechanical 
processes  has  failed,,  the  compound  yields  to  the  influence 
of  some  of  the  so-called  physical  forces,  as  heat,  light, 
electricity.  This  leads  to  the  conclusion  that  there  are 
at  least  two  varieties  of  compound  substances.  Each  of 
these  varieties  is  more  or  less  strongly  characterized  by 
external  properties.  As  regards  those  which  can  be  de- 
composed, by  mechanical  means  alone,  it  is  true  that  they 
possess  the  combined  properties  of  their  constituents; 
and  these  constituents  are  usually  contained  in  the  com- 
pound in  their  original  forms.  As  regards  the  second 
class  of  compounds,  on  the  other  hand,  it  is  just  as  true 
that  they  do  not  possess  the  properties  of  their  constitu- 
2 


14          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

ents;  and  these  constituents  are  contained  in  the  com- 
pounds in  forms  differing  entirely  from  those  originally 
possessed  by  them. 

Chemism. — Substances  which  are  held  together  by 
cohesion  or  adhesion  can  be  separated  by  mechanical 
means.  But  we  have  here  evidence  of  the  existence  of 
some  force  which  holds  substances  together,  and  which 
cannot  be  overcome  by  mechanical  means.  To  this  force 
the  name  chemical  affinity  or  chemism  has  been  given. 
The  object  of  the  science  of  chemistry  is  the  study  of  this 
force  in  its  relations  to  matter;  or  the  study  of  the  action 
of  matter  upon  matter,  as  far  as  it  is  influenced  by  this 
force.  In  regard  to  the  position  which  chemistry  occu- 
pies among  the  sciences,  it  will  be  seen  that  it  is  primarily 
a  branch  of  physics,  if  to  the  latter  science  is  given  its 
broadest  scope.  But,  further,  chemism  always  gives  rise 
to  the  formation  of  new  bodies,  and  the  study  of  these  in 
their  relations  to  each  other  becomes  a  legitimate  part  of 
the  object  of  chemistry.  This  allies  the  science  to  that 
branch  of  study  embraced  under  the  head  Natural  His- 
tory. Owing  to  this  twofold  character,  it  is  customary 
to  treat  the  subject  as  forming  an  independent  science, 
and,  for  many  reasons,  this  is  the  most  convenient  method. 

We  have  thus  far  recognized  the  existence  of  the  force, 
chemism  ;  and  also  become  acquainted  with  one  of  the 
characteristics  of  its  action.  The  knowledge  of  the  force 
remained  for  a  long  time  in  this  state,  as  the  methods  of 
investigation  at  first  employed  could  not  disclose  its 
most  important  characteristics.  Up  to  the  latter  part  of 
the  eighteenth  century,  the  qualitative  method  of  investi- 
gation was  of  necessity  the  principal  one  employed  in 
the  study  of  chemical  phenomena ;  that  is  to  say,  the 
quality  of  the  substances  allowed  to  act  upon  each  other, 
and  the  quality  of  the  product  or  products  were  noted, 
but  little  attention  being  given  to  the  amounts  of  the 
substances  employed,  or  of  those  obtained  as  products. 
As  the  importance  of  the  quantitative  method  became 
more  and  more  apparent,  the  means  for  applying  it  also 
gradually  made  their  appearance.  The  balance,  the  sine 
qua  non  of  chemistry,  was  improved,  and,  finally,  in 
Lavoisier's  hands  led  to  tangible  results.  To  its  use  is 
to  be  ascribed  the  correct  explanation  of  the  phenomenon 


ATOMIC    THEORY — ATOMIC    WEIGHTS,    ETC.         15 

of  combustion,  a  phenomenon  which,  considered  in  all  the 
varied  forms  in  which  it  is  presented  to  us,  must  be 
looked  upon  as  the  most  important  of  all  chemical  phe- 
nomena. But,  though  the  explanation  of  combustion  was 
correctly  given,  no  new  property  of  chemism  was  dis- 
covered. A  new  example  of  the  kind  of  action  which 
was  already  known  to  characterize  the  force  was  added 
to  the  list  f  the  key  was  given  to  a  better  understanding 
of  the  chemical  nature  of  gaseous  bodies;  the  indestructi- 
bility of  matter  became,  perhaps,  more  distinctly  evident 
than  it  had  hitherto  been ;  but  the  knowledge  of  chemism 
as  such  remained  what  it  had  been  up  to  that  time.  That 
a  definite  result  was  obtained  through  a  consideration  of 
the  quantitative  relations  of  an  experiment  was  the  fact 
which,  above  all  others,  gave  an  impulse  to  the  subse- 
quent development  of  the  science. 

Investigations  of  chemical  phenomena  now  took,  in 
general,  a  different  direction ;  and  soon  the  united  work 
of  many  hands  succeeded  in  establishing  a  fundamental 
principle  of  the  science.  The  first  semblance  of  a  gene- 
ral law  governing  chemical  action  made  its  appearance 
when  it  was  finally  established  beyond  a  doubt  that  the 
combination  of  bodies,  under  the  influence  of  chemism, 
always  takes  place  in  fixed  proportions.  This  principle, 
though  perhaps  tacitly  acknowledged  by  many  chemists, 
was  not  fully  established  until  the  beginning  of  the  pre- 
sent century.  In  1803,  a  strong  effort  was  made  by 
Berthollet,  in  his  work  entitled  "  Statique  Chimique,"  to 
prove  the  incorrectness  of  the  principle,  but  the  oppo- 
sition called  forth  by  this  work,  particularly  from  Proust, 
led  to  more  and  more  careful  examinations  of  the  so-called 
chemical  compounds,  and  thus  to  the  firm  establishment 
of  the  principle.  Proust  also  showed  that  two  bodies 
could  combine  with  each  other  in  more  than  one  propor- 
tion, and  that  for  each  combination  the  relative  propor- 
tions of  the  constituents  were  fixed. 

Dalton's  Investigations. — In  the  year  1804,  Dalton's 
investigations  enabled  him  to  take  another  advance-step. 
Another  general  law  governing  chemical  action  was  dis- 
covered and  propounded.  This  was  the  law  of  multiple 
proportions.  As  this  is  the  foundation  of  the  science,  as 
it  is  at  present,  let  us  follow,  somewhat  in  detail,  Dalton's 


16          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

reasoning.  Many  substances  had  been  analyzed  before 
his  time,  and  the  percentages  of  the  constituents  had 
been  determined  with  a  tolerable  degree  of  accuracy. 
He  examined  first  two  gases,  both  of  which  consist  of 
carbon  and  hydrogen,  viz.,  olefiant  gas  and  marsh-gas. 
He  analyzed  them  both,  and  determined  the  percentage  of 
the  constituents  contained  in  them.  These  percentages 
were  as  follows : — 

Olefiant  gas,  85.7  per  cent,  carbon,  and  14.3  per  cent,  hydrogen. 
Marsh-gas,  75.0  "  "  25.0  " 

On  comparing  these  numbers  he  found  that  the  ratio 
of  carbon  to  hydrogen  in  olefiant  gas  was  as  6  to  1  ; 
whereas,  in  marsh-gas  it  was  3  to  1,  or  6  to  2.  The 
amount  of  hydrogen  combined  with  a  given  amount  of 
carbon  was  exactly  twice  as  great  in  the  one  case  as  in 
the  other. 

For  the  two  oxides  of  carbon,  further,  the  following 
numbers  were  obtained  : — 

Carbon  monoxide,  42.86  p.  c.  carbon,  and  57.14  p.  c.  oxygen. 
Carbon  dioxide,      27.27         "  72.73 

But  42.86  :  57.14  :  :  6  :  8,  and  27.27  :  72.73  :  :  6  :  16. 
The  amount  of  oxygen  combined  with  a  given  amount 
of  carbon  in  carbon  dioxide  was  exactty  twice  as  great  as 
the  amount  of  oxygen  combined  with  the  same  amount 
of  carbon  in  carbon  monoxide.  He  saw  again  that,  in 
olefiant  gas,  one  part  by  weight  of  hydrogen  combined 
with  six  parts  by  weight  of  carbon ;  and  that  in  carbon 
monoxide  eight  parts  by  weight  of  oxygen  combined  also 
with  six  parts  by  weight  of  carbon.  Water  was  now 
examined..  It  contains  88.89  per  cent,  oxygen  and  11.11 
per  cent,  hydrogen  ;  and  these  numbers  are  to  each  other 
as  8  to  1.  The  numbers  which,  in  the  first  place,  repre- 
sented the  combining  proportions  of  oxygen  and  hydro- 
gen respectively  with  carbon,  are  also  found  to  represent, 
in  the  second  place,  the  combining  proportions  of  oxygen 
and  hydrogen  with  each  other.  All  subsequent  exami- 
nations of  other  compounds  led  to  similar  results,  and 
thus  Dalton  had  discovered  the  law  of  multiple  propor- 
tions. This  may  be  stated  as  follows  :• — 

If  two  bodies,  A  and  B,form  several  compounds 
with  each  other,  and  we  consider  any  fixed  amount 


ATOMIC    THEORY  —  ATOMIC    WEIGHTS,    ETC.         17 

of  A,  then  the  different  amounts  of  B  which  com- 
bine  with  this  fixed  amount  of  A  bear  a  simple  ratio 
to  each  other. 

This  is  a  fixed  law,  and  it  was  generally  acknowledged 
as  such  by  contemporaneous  chemists.  Thus  another 
characteristic  of  chemism  was  clearly  pointed  out. 

Atomic  Theory, — But  Dalton  did  not  stop  with  the 
discovery  of  the  law  of  multiple  proportions ;  he  sought 
for  its  explanation.  He  was  thus  led  to  propose  the 
atomic  theory,  as  affording  the  simplest  explanation  of 
the  facts  as  observed. 

The  question  as  to  the  ultimate  constitution  of  matter 
had  frequently  and  from  the  earliest  dates  been  discussed. 
Two  views  were  held  at  different  periods  and  by  different 
thinkers.  One  of  these  supposed  matter  to  be  indefinitely 
divisible.  The  other  supposed  that  there  was  a  limit  to 
the  divisibility,  and  that  this  limit  was  reached  when  the 
division  had  been  carried  down  to  certain  small  particles 
called  atoms.  After  the  discovery  of  the  law  of  multiple 
proportions,  however,  the  atomic  theory  was  further 
developed,  and,  in  consequence,  acquired  a  more  definite 
form,  as  the  existence  of  atoms  was  supposed  to  have  a 
direct  connection  with  chemical  combinations.  The  results 
of  Dalton's  investigations  are  not  fully  stated  in  the  law 
of  multiple  proportions  as  above  given  ;  another  fact  was 
made  clear  which  is  also  of  importance.  The  complete 
results  may  be  stated  as  follows :  It  was  shown  that  for 
each  element  a  particular  number  might  be  selected  ;  and 
that  this  number  or  a  simple  multiple  of  it  would  repre- 
sent the  proportion  by  weight  in  which  this  element  com- 
bined with  other  elements.  Dalton  explained  this  by 
supposing  that  chemical  action  takes  place  between 
atoms,  i.  e.,  between  particles  that  are  indivisible  and 
have  definite  weights.  If  chemical  combination  takes 
place  between  one  atom  of  one  substance  and  one  atom 
of  another  substance,  or  between  a  simple  number  of 
atoms  of  one  substance  and  a  simple  number  of  atoms  of 
another,  and  "these  atoms  have  definite  weights,  then 
indeed  the  explanation  of  the  laws  of  definite  and  mul- 
tiple proportions  is  given. 

Thus  the  idea  of  atoms  became  a  much  more  tangible 
2* 


18          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

one  than  it  had  been  up  to  that  time.  Not  only  were 
atoms  supposed  to  have  definite  weights,  but  a  method 
was  given  by  means  of  which  their  relative  weights  could 
be  determined.  The  number  assigned  to  an  element, 
representing  its  combining  proportion,  would  also  repre- 
sent the  relative  weight  of  its  atom.  The  fact  that  the 
combining  proportion  of  an  element  was  in  some  cases 
represented  hy  a  multiple  of  the  simplest  number  was 
satisfactorily  accounted  for  by  supposing  that  in  these 
cases  more  than  one  atom  of  the  element  combined  with 
one  atom  of  another  element. 

Determination  of  Atomic  Weights. — The  determination 
of  atomic  weights  became  now  the  chief,  immediate  prob- 
lem of  the  science  of  chemistry.  Dalton's  atomic  theory 
was  accepted  by  many,  though  not  by  all.  The  laws 
governing  chemical  combinations  could  not  be  doubted, 
but  the  explanation  could  be  and  was.  Nevertheless,  the 
importance  of  determining  for  each  element  the  character- 
izing number,  call  it  atomic  weight  or  combining  propor- 
tion, was  acknowledged  by  all ;  and  consequently  par- 
ticular attention  was  given  to  this  field  of  research  during 
the  period  directly  following  Dalton's  publication.  Let 
us  see  how  thoroughly  the  desired  object  could  be  accom- 
plished alone  by  the  aid  of  the  principles  laid  down  by 
Dalton. 

At  the  time  of  which  we  are  speaking,  the  methods  for 
chemical  analysis  wrere  still  far  from  perfect,  and  hence 
most  of  the  determinations  then  made  required  subse- 
quent corrections  which  were  gradually  forthcoming  as 
the  analytical  methods  were  improved.  This  fact,  how- 
ever, has  nothing  to  do  with  the  subject  under  consider- 
ation. The  principle  alone  is  involved.  The  question  is 
to  be  answered :  Can  we,  on  logical  grounds,  with  the 
principles  contained  in  Dalton's  investigations,  ever  deter- 
mine the  relative  weights  of  the  atoms  of  elements  ?  To 
decide  this  question  we  must  first  examine  more  carefully 
Dalton's  method  for  determining  atomic  weights.  In  the 
following  brief  discussion  the  correct  numbers,  as  given 
by  later  analyses,  are  employed,  instead  of  those  origi- 
nally found.  This  does  not  interfere  with  the  principle, 
and  does  simplify  the  matter  otherwise. 


ATOMIC    THEORY  —  ATOMIC    WEIGHTS,    ETC.         19 

Method  for  the  Determination  of  Atomic  Weights  de- 
pendent upon  Analysis. — As  the  standard  the  combining 
number  of  hj'drogen  was  first  selected,  and  this  made  1. 
Hydrogen  combines  with  oxygen  in  the  proportion  of 
1:8;  and  as  water  was  the  only  known  compound  of 
hydrogen  and  oxygen,  the  conclusion  was  drawn  that  the 
two  elements  were  united  atom  to  atom,  and  hence  the 
atomic  weight  of  oxygen  was  8.  Further,  nitrogen  is 
combined  with  hydrogen  in  ammonia  in  the  proportion 
of  1  part  by  weight  of  hydrogen  to  4f  parts  by  weight 
of  nitrogen.  Ammonia  was  the  only  compound  of  nitro- 
gen and  hydrogen  known  ;  and  the  same  reasoning,  as 
above  employed,  led  to  the  conclusion  that  the  atomic 
weight  of  nitrogen  was  4f.  Considering  for  a  moment 
these  two  simple  cases,  we  see  that  the  numbers  thus 
found,  as  representing  the  relative  weights  of  the  atoms 
of  oxygen  and  nitrogen,  are  founded  partially  upon 
hypothesis.  There  is  nothing  to  decide  as  to  the  number 
of  atoms  of  hydrogen  or  oxygen  that  are  contained  in 
water,  nor  of  nitrogen  and  hydrogen  in  ammonia,  and, 
of  course,  as  long  as  this  number  is  unknown,  it  is  impos- 
sible to  draw  any  positive  conclusion  with  reference  to 
the  atomic  weights  of  nitrogen  and  oxygen.  Any  con- 
clusion thus  drawn  is  dependent  upon  a  thorough  knowl- 
edge of  the  compounds  of  the  particular  element  under 
consideration.  Such  a  number  must  finally  be  selected 
as  is  most  in  accordance  with  the  facts.  This  selection 
must  remain  more  or  less  arbitrary,  as  may  be  more 
clearly  and  decidedly  shown. 

Take  again  the  case  of  oxygen.  A  second  compound 
of  hydrogen  and  oxygen  is  vnow  known  containing  the 
elements  in  the  proportion  1  :  16.  At  first  sight,  the 
explanation  of  this  may  appear  simple  enough.  In  this 
second  compound  there  are  two  atoms  of  oxygen  com- 
bined with  one  of  hydrogen,  and  thus  the  proportion  is 
satisfied.  But  may  we  not  with  equal  right  decide  that 
in  water  there  are  two  atoms  of  hydrogen  combined  with 
one  of  oxygen  ?  This  would  give  us  for  oxygen  the 
atomic  weight  1 6,  and,  in  the  second  compound,  we  would 
have  contained  one  atom  of  each  of  the  elements. 

Further,  if  we  attempt  to  determine  the  atomic  weight 
of  carbon  by  Dalton's  method,  we  shall  encounter  diffi- 
culties fully  as  great,  and  our  final  selection  among  many 


20          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

numbers  will  be  arbitrary.  Taking  olefiant  gas,  we  have 
hydrogen  combined  with  carbon  in  the  proportion  1:6; 
in  marsh-gas  the  proportion  of  the  same  constituents  is 
1  :  3  or  2  :  6.  If  we  suppose  that  in  olefiant  gas  the 
elements  are  combined  atom  with  atom,  then  the  atomic 
weight  of  carbon  would  be  6,  and  consequently  in  marsh- 
gas  we  would  have  two  atoms  of  hydrogen  combined 
with  one  atom  of  carbon.  But  here  again  we  can  just  as 
well  suppose  that  in  marsh-gas  we  have  the  simplest  kind 
of  combination,  and  this  would  give  us  for  the  atomic 
weight  of  carbon  3.  Then  in  olefiant  gas  two  atoms  of 
carbon  would  be  combined  with  one  atom  of  hydrogen. 

Finally,  let  us  take  the  oxygen  compound  of  carbon. 
In  carbon  monoxide,  carbon  is  united  with  oxygen  in  the 
proportion  of  6  :  8  or  3  :  4 ;  whereas  in  carbon  dioxide 
the  corresponding  proportion  is  3  :  8  or  6  :  16.  Now 
let  us  suppose  the  atomic  weight  of  ox}?gen  to  be  equal 
to  8,  then,  if  carbon  monoxide  is  the  simpler  of  the  two 
compounds,  the  atomic  weight  of  carbon  is  6 ;  and  in 
carbon  dioxide  there  are  two  atoms  of  oxygen  combined 
witli  each  atom  of  carbon.  Here,  again,  it  is  evident 
that  we  can  just  as  well  imagine  carbon  dioxide  to  be  the 
simpler  compound,  in  which  case  the  atomic  weight  of 
carbon  would  be  3,  and  in  carbon  monoxide  there  would 
be  two  atoms  of  carbon  combined  with  one  atom  of  oxygen. 
Between  these  different  possibilities  it  is  impossible  to 
draw  a  logical  conclusion  with  the  aid  of  the  knowledge 
which  can  be  obtained  by  analysis.  The  number  of  similar 
instances  might  be  increased  indefinitely  ;  the  inadequacy 
of  the  method  could  be  made  more  strikingly  clear  by 
examples  of  a  more  complicated  kind,  but  the  cases  men- 
tioned are  sufficient  for  our  purpose ;  we  are  obliged  to 
look  for  other  methods  for  the  determination  of  atomic 
weights  if  we  would  free  the  numbers  from  arbitrariness. 

Equivalents. — This  necessity  was  recognized  first  and 
most  clearly  by  Wollaston  in  1814.  As  no  method  pre- 
sented itself  to  him  which  would  furnish  a  firm  founda- 
tion for  the  determination  of  atomic  weights,  he  proposed 
to  abandon  the  idea  of  atomic  weights  entirely,  and  to 
substitute  for  it  that  of  the  equivalent,  thus,  as  he  sup- 
posed, getting  rid  of  all  hypotheses  and  obtaining  numbers 


ATOMIC    THEORY  —  ATOMIC    WEIGHTS,    ETC.         21 

that  would  be  the  simple  expressions  of  proved  facts. 
The  equivalent  of  an  element  was  to  him  that  quantity 
of  the  element  that  possessed  the  same  chemical  value*  as 
a  given  quantity  of  another  element,  that  quantity  of  an 
element  that  could  play  the  same  role  as  a  given  quantity 
of  another  element.  According  to  the  conditions  of  this 
definition,  it  is  plain  that,  in  order  to  know  what  portions 
of  two  elements  are  equivalent,  we  must  be  able  to  com- 
pare the  two.  Hence,  primarily,  only  of  such  elements 
as  can  be  compared  with  each  other,  of  such  as  possess  a 
certain  degree  of  similarity,  can  the  equivalent  quantities 
be  determined.  As  this  direct  comparison  is  not  always, 
nor,  indeed,  in  the  majority  of  cases,  possible,  recourse 
must  be  had  to  indirect  comparison. 

To  illustrate  this  let  us  take  an  example.  Hydrogen 
and  chlorine  combine  with  each  other  in  the  proportion 
of  1  part  by  weight  of  hydrogen  to  35.5  parts  by  weight 
of  chlorine,  and  from  this  fact  we  draw  the  conclusion 
that  35.5  parts  of  chlorine  are  equivalent  to  1  part  of 
hydrogen.  We  find  in  the  same  way  that  8  parts  of 
oxygen,  80  of  bromine,  16  of  sulphur  are  all  equivalent 
to  1  part  of  hydrogen.  Knowing  that  35.5  represents 
the  equivalent  of  chlorine,  we  determine  the  quantities  of 
sodium  and  silver  that  are  respectively  equivalent  to  this 
quantity  of  chlorine.  We  find  for  sodium  23  and  for 
silver  108.  These  quantities  of  silver  and  sodium  are 
further  found  to  be  equivalent  to  8  parts  of  oxygen,  80 
parts  of  bromine,  and  16  parts  of  sulphur,  and  hence  we 
conclude  that  they  are  also  equivalent  to  1  part  of  hydro- 
gen. Thus  the  equivalents  of  sodium  and  silver  have 
been  determined  by  the  method  of  indirect  comparison. 
Sodium  and  silver  do  not  combine  with  Irydrogen,  yet 
the  equivalent  numbers  found  are  intended  to  express  the 
proportions  in  which  they  would  combine  with  hydrogen, 
provided  such  combination  were  possible.  We  are  amply 
justified  in  this  in  most  simple  cases,  but  nevertheless  it 
must  be  distinctly  borne  in  mind  that  such  numbers  as 
are  determined  by  indirect  comparison  with  the  standard, 
whatever  this  may  be,  are  not  in  the  strictest  sense 
expressions  of  proved  facts ;  the  last  step  in  the  deter- 
minations, however  justified  we  may  be  in  taking  it, 
requires,  nevertheless,  the  aid  of  hypothesis. 

But  if  the  difficult}7  thus  referred  to  were  the  only  one 


22          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

to  be  met  with  in  the  determination  of  equivalent  numbers, 
such  determinations  would  have  nearly  the  full  value 
claimed  for  them  by  Wollaston.  This,  however,  is  not 
the  case.  As  soon  as  we  consider  any  but  the*  simplest 
forms  of  compounds,  we  are  left  in  fully  as  much  doubt 
in  regard  to  the  equivalent  numbers  as  we  were  in  regard 
to  atomic  weights.  If  it  be  required  to  determine  the 
quantity  of  carbon  that  is  equivalent  to  1  part  of  hydro- 
gen, the  compounds  of  the  two  elements  must  be  examined. 
But  there  are  a  great  many  compounds  of  these  two  ele- 
ments. Taking  but  two,  olefiant  gas  and  marsh-gas,  we 
find  that  in  the  former  (see  ante,  p.  16)  1  part  of  hydrogen 
is  combined  with  (equivalent  to)  6  parts  of  carbon ; 
whereas,  in  the  latter,  1  part  of  hydrogen  is  combined 
with  (equivalent  to)  3  parts  of  carbon.  What  shall  here 
decide  which  is  the  correct  number?  It  is  evident  from 
such  instances  as  this  that  the  idea  of  equivalent  is  fully 
as  uncertain  as  that  of  the  atom  was  at  the  time  we  are 
considering.  That  an  element  could  be  equivalent  to  two 
entirely  different  quantities  was  in  itself  somewhat  para- 
doxical, if  the  original  definition  of  equivalent  was  re- 
tained. These  difficulties  seem  not  to  have  been  apparent 
to  Wollaston.  He  continued  his  determinations  of  equi- 
valents, and  during  this  time  a  fusion  of  the  ideas  of 
equivalent  and  atomic  weight  took  place  unconsciously. 
As  neither  of  these  ideas  was  then  definite,  as  to  each  of 
a  number  of  elements  a  number  of  atomic  weights  could 
be  assigned,  and  almost  as  many  equivalents,  the  suc- 
ceeding period  in  the  history  of  chemistry  presents  a 
disagreeably  confused  condition,  until  it  became  felt  on 
all  sides  that  some  new  idea  or  ideas  must  be  introduced, 
if  a  fair  foundation  for  the  science  was  to  be  reached. 

Determinations  by  Berzelius. — Before  the  necessary 
new  ideas  were  introduced,  the  methods  at  hand  were 
employed  to  the  full  extent.  All  known  compounds  of 
any  given  element  were  compared  with  each  other,  and  a 
number  finally  selected,  that  would  best  satisfy  the  facts, 
to  represent  the  equivalent  of  the  element,  or  its  atomic 
weight,  as  it  was  called  by  others.  Berzelius  attacked 
the  subject  most  successfully.  He  laid  down  rules,  by 
the  aid  of  which  the  number  of  atoms  of  an  element  con- 
tained in  a  compound  could  be  determined,  and  hence 


ATOMIC    THEORY  —  ATOMIC    WEIGHTS,    ETC.         23 

also  its  atomic  weight.  Then,  by  more  careful  analyses 
than  had  been  previously  made,  the  atomic  weights  or 
equivalents  of  all  the  elements  were  determined.  A  large 
number  o'f  these  determinations  depended  for  their  cor- 
rectness upon  chemical  rules,  similar  to  the  following, 
given  by  Berzelius  : — 

If  an  element  forms  several  oxides,  and  the  quan- 
tities of  oxygen  contained  in  them,  as  compared  with 
a  fixed  quantity  of  the  element,  bear  the  proportion 
1  :  2,  then  it  is  to  be  concluded  that  the  first  com- 
pound consists  of  one  atom  of  the  element  and  one 
atom  of  oxygen ;  the  second,  of  one  atom  of  the 
element  and  two  atoms  of  oxygen  (or  two  atoms  of 
the  element  and  four  atoms  of  oxygen).  If  the  pro- 
portion is  2  :  3,  then  the  first  compound  consists  of 
one  atom  of  the  element  and  two  atoms  of  oxygen; 
the  second,  of  one  atom  of  the  element  and  three 
atoms  of  oxygen,  etc. 

This  rule  covers  those  cases  in  which  it  is  required  to 
determine  the  atomic  weight  of  an  element  by  a  consider- 
ation of  its  oxides.  Other  rules  were  given  in  which 
sulphur  compounds,  etc.,  were  made  the  basis  of  calcu- 
lation. 

It  will  be  observed  that,  although  in  these  rules  the 
ox^ygen  and  sulphur  are  taken  as  the  elements,  the  number 
of  whose  atoms  varies,  the  other  elements  might  theoreti- 
cally be  considered  in  the  same  way,  and  the  atomic 
weights  obtained  would  then  be  entirely  different.  An 
example  will  make  this  clear:  Mercury  combines  with 
oxygen  in  two  proportions.  In  the  first  compound,  8 
parts  of  oxygen  are  combined  with  200  parts  of  mercury  ; 
in  the  second,  16  parts  of  oxygen  are  combined  with  200 
parts  of  mercury.  Adopting  the  rule  above  laid  down, 
we  would  conclude  that,  in  the  first  compound,  1  atom  of 
mercury  is  combined  with  1  atom  of  oxygen ;  and  in  the 
second,  1  atom  of  mercury  with  2  atoms  of  oxygen.  If, 
then,  8  is  the  atomic  weight  of  oxygen,  200  is  the  atomic 
weight  of  mercury.  But  if,  on  the  other  hand,  we  con- 
sider the  quantity  of  oxygen  as  remaining  fixed,  and  that 
of  the  mercury  as  varying,  then  we  would  have  in  the 
first  compound,  8  parts  of  oxygen  combined  with  200 
parts  of  mercuiy,  and  in  the  second,  8  parts  of  oxygen 
combined  with  100  parts  of  mercury ;  and,  by  a  similar 


24          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

process  of  reasoning,  we  might  draw  the  conclusion  that 
the  first  compound  contains  2  atoms  of  mercury  to  1  atom 
of  oxygen,  and  the  second,  1  atom  of  mercury  to  1  atom 
of  oxygen;  and  thus  we  would  obtain  100  as  the  atomic 
weight  of  mercury  instead  of  200,  as  found  above.  Ber- 
zelius  had  made  certain  observations  on  chemical  bodies 
upon  which  he  based  his  rules,  but,  as  we  shall  see,  these 
observations  were  not  sufficient. 

Another  difficulty  presented  itself  in  the  case  of  those 
elements  that  only  combined  in  one  proportion  with 
oxygen.  What  should  decide  in  regard  to  the  number 
of  atoms  of  oxygen  contained  in  a  compound  of  such  an 
element?  Here  speculation  was  the  only  aid,  and  it 
often  led  to  false  results. 

The  Principle  of  Substitution  employed  in  the  Deter- 
mination of  Atomic  Weights. — The  researches  of  Ber- 
zelius  added  a  vast  amount  to  the  knowledge  of  the  com- 
bining weights  of  the  elements,  and  it  must  be  acknow- 
ledged that  the  determinations  made  by  him  rested  upon 
a  somewhat  firmer  basis  than  the  determinations  made 
previous^.  He  made  the  fullest  and  most  logical  use  of 
purely  chemical  means  that  could  be  made  at  the  time. 
Subsequently,  however,  a  new  fact  was  discovered  in 
connection  with  chemical  compounds  which  proved  of 
great  value  in  simplifying  the  consideration  of  chemical 
phenomena,  and  also  aided  materially  in  the  solution  of 
the  problem  of  the  determination  of  atomic  weights. 
This  is  substitution.  The  subject  will  be  considered 
more  fully  in  the  last  section ;  here  a  brief  explanation 
will  suffice  for  the  purpose  of  exhibiting  its  connection 
with  the  problem  with  which  we  are  at  present  dealing. 
It  has  been  found  that  certain  elements  have  the  power 
of  entering  into  compound  bodies,  driving  out  some  of 
the  constituents,  and  taking  the  place  thus  left  vacant. 
For  instance,  water  contains  2  atoms  of  hydrogen  and 
one  of  oxygen ;  if  we  allow  potassium  to  act  upon  water, 
a  portion  of  the  hydrogen  is  given  off,  and  a  new  coin- 
pound  containing  both  potassium  and  hydrogen,  in  ad- 
dition to  the  oxygen,  is  the  result.  If  now  potassium 
be  further  allowed  to  act  upon  this  new  compound,  the 
hydrogen  contained  in  it  will  be  driven  out,  and  its  place 


ATOMIC    THEORY  —  ATOMIC    WEIGHTS,    ETC.         25 

will  be  taken  by  potassium.  Thus  we  obtain  from  water, 
by  replacing  its  hydrogen  by  potassium,  a  compound 
containing  2  atoms  of  potassium  and  1  atom  of  oxygen. 
This  kind  of  action  is  called  substitution. 

To  show  how,  by  taking  into  account  the  transform- 
ations included  under  this  head,  we  may  draw  conclu- 
sions of  importance  with  reference  to  atomic  weights,  one 
simple  example  may  suffice:  We  have  seen  that  the  chief 
difficulty  in  determining  atomic  weights  or  equivalents  by 
chemical  means  consists  in  the  lack  of  data  for  estimating 
the  number  of  atoms  of  an  element  contained  in  any  given 
compound.  Considering  marsh-gas,  we  find  that  in  it  1 
part  of  hydrogen  is  combined  with  3  parts  of  carbon, 
and,  as  above  stated,  we  might  conclude  from  this  fact 
that  the  atomic  weight  of  carbon  is  3.  If,  however,  we 
can  by  any  means  prove  that  there  are  more  than  one 
atom  of  hydrogen  contained  in  the  gas,  the  conclusion 
would  require  modification.  By  means  of  the  process  of 
substitution,  this  can  be  proved.  By  allowing  chlorine 
to  act  upon  marsh-gas  under  proper  conditions,  a  portion 
of  the  hydrogen  will  be  replaced,  and  a  compound  con- 
taining carbon,  hydrogen,  and  chlorine  will  result.  This 
new  compound  treated  with  chlorine  again  gives  up  a 
portion  of  its  hydrogen,  and  takes  up  chlorine  in  its 
place.  This  operation  may  be  repeated  four  times,  and 
thus  finally  a  compound  is  obtained  which  contains  only 
carbon  and  chlorine.  Each  time  the  same  amount  of 
hydrogen  is  given  up,  and  is  replaced  by  an  equivalent 
amount  of  chlorine.  Thus  it  is  plain  that  the  hydrogen 
originally  contained  in  marsh-gas  is  divisible  into  four 
parts,  and  we  are  obliged  to  accept  the  conclusion  that 
there  are  at  least  four  atoms  of  hydrogen  contained  in 
marsh-gas — a  conclusion  which  we  could  not  possibly 
reach  by  the  aid  of  the  means  heretofore  considered.  If 
now  we  take  that  amount  of  carbon  which  is  in  combina- 
tion with  four  atoms  of  hydrogen  as  representing  one 
atom  (and,  by  a  consideration  of  the  whole  list  of  carbon 
compounds,  we  are  justified  in  this  step),  then  the  atomic 
weight  of  carbon  is  12.  The  method  thus  briefly  illus- 
trated is  capable  of  application  to  a  considerable  extent, 
but  not  to  such  an  extent  as  to  render  it  a  general 
method  for  the  determination  of  atomic  weights. 
3 


26          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

Consideration   of   Chemical  Decompositions  for  the 

purpose   of  determining   Atomic    Weights One    more 

method  of  reasoning  must  be  referred  to  as  having  been 
employed,  either  for  the  purpose  of  furnishing  proofs  of 
the  correctness  of  atomic  weights  determined  by  other 
means,  or  for  the  direct  determination  of  these  weights. 
An  example  will  best  make  this  matter  clear.  We  wish 
to  know,  for  instance,  how  many  atoms  of  hydrogen  are 
combined  with  nitrogen  in  ammonia ;  or,  having  by  the 
preceding  method  concluded  that  this  number  is  3,  we 
wish  to  verify  the  conclusion  by  other  observations.  By 
treating  nitric  acid  (which  we  will  suppose  to  contain 
one  atom  of  hydrogen  to  every  atom  of  nitrogen)  with 
hydrogen,  we  obtain  ammonia.  Now,  if  we  consider  the 
amount  of  hydrogen  that  in  nitric  acid  was  in  combina- 
tion with  the  nitrogen,  we  find  that,  in  the  resulting 
ammonia,  three  times  as  much  hydrogen  is  combined 
with  the  same  amount  of  nitrogen.  Further,  ammonia 
combines  directly  with  a  number  of  compounds,  and,  if 
we  examine  the  amount  of  hydrogen  contained  in  this 
ammonia,  we  find  that  it  must  necessarily  be  represented 
with  three  or  some  multiple  of  three  atoms  of  hydrogen. 
Thus,  if  we  study  the  various  cases  in  which  ammonia  is 
either  formed,  or  destroyed,  or  enters  into  combination,  we 
find  alwa\*s  that  the  quantity  of  ammonia  thus  playing  a 
part  must  contain  three  or  some  multiple  of  three  atoms 
of  hydrogen  ;  and  hence  we  are  again  led  to  the  conclu- 
sion that,  in  ammonia  at  least,  three  atoms  of  hydrogen 
are  combined  witli  every  atom  of  nitrogen. 


The  methods  we  have  thus  briefly  described  comprise 
all  we  have  at  our  command  for  the  determination  of 
atomic  weights  dependent  upon  purely  chemical  pro- 
cesses. Consider  these  methods  as  we  may,  we  must  see 
that  they  are  inadequate  to  the  accomplishment  of  their 
object.  The  determinations  may  indeed  be  made,  but  at 
last  there  must  always  remain  a  doubt  concerning  the 
result.  If  then  we  can  approach  the  subject  from  an 
entirely  different  direction,  we  shall  succeed  in  reducing 
this  doubt  to  a  minimum,  if  we  find  that  the  results  at 
first  obtained  assert  themselves  as  correct  in  the  second 
instance.  Before  passing,  however,  to  a  consideration  of 


ATOMIC    THEORY  —  ATOMIC!    WEIGHTS,    ETC.         27 

new  methods  for  making  these  determinations,  it  will  be 
well  to  apply  the  knowledge  we  have  gained  in  fixing 
more  definitely  than  lias  yet  been  done  the  ideas  of  ele- 
ments and  compounds. 

Elements. — The  theoretical  idea  of  an  element  has 
already  been  stated.  An  element,  strictly  speaking,  is  a 
substance  that  cannot  by  any  possible  means  be  decom- 
posed into  kinds  of  matter  that  are  unlike.  This  defini- 
tion presupposes  a  knowledge  of  all  possible  means  for 
decomposing  bodies.  Until  we  are  positive  that  we  are 
acquainted  with  all  these  means,  we  cannot  be  positive 
in  regard  to  the  existence  of  a  single  element.  But  it  is 
plain  that  to  assert  the  possession  of  this  amount  of 
knowledge  would  be  in  the  highest  degree  presumptuous. 
We  can  then  never  assert  positively  that  any  given  sub- 
stance is  an  element ;  we  can  only  say  that,  the  means  at 
our  command  being  insufficient  to  bring  about  the  de- 
composition of  a  given  bodj^,  we  consider  this  substance 
an  element  until  such  time  shall  arrive  when,  new  means 
being  given,  it  shall  be  shown  to  be  compound.  Nume- 
rous instances  of  the  change  of  opinion  concerning  the 
elementary  character  of  different  substances  might  be 
adduced,  prominent  among  which  would  be  the  alkaline 
metals,  the  oxides  of  which  were  for  a  time  looked  upon 
as  elements  ;  chlorine,  which  was  looked  upon  as  a  com- 
pound body  until  it  had  been  satisfactorily  shown  that 
we  were  not  in  possession  of  the  means  for  decomposing 
it,  etc.  etc.  Thus  the  number  of  elements,  as  stated  at 
any  given  time,  is  entirely  dependent  upon  the  state  of 
chemical  analysis  at  that  time,  and  is  never  an  expression 
of  an  absolute  fact.  At  present,  the  number  of  elements 
known  is  64.  In  other  words,  we  can  recognize  the 
existence  of  64  different  kinds  of  matter. 

If  we  consider  the  atoms  which  make  up  an  elementary 
substance,  we  see  that  they  must  necessarily  be  of  the 
same  kind,  how  far  soever,  we  consider  the  subdivision 
of  the  substance  as  taking  place  before  the  atom  is 
reached.  Accepting  then  the  existence  of  atoms,  an 
element  may  be  defined  as  a  substance  made  up  of  atoms 
of  the  same  kind  ;  and  we  shall  see  that  the  definition  of 
an  atom,  which  will  be  given  further  on,  makes  this  defi- 
nition of  an  element  a  strict  one  in  every  respect. 


28          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

Compounds. —  It  has  been  stnted  that  observation 
shows  us  the  existence  of  at  least  two  varieties  of  com- 
pound bodies.  To  only  one  of  these,  however,  is  the 
name  compound  strictly  applied,  and  then  the  name 
signifies  a  chemical  compound.  To  the  other  class 
various  names  are  applied,  according  to  the  nature  of  the 
substance,  such,  for  instance,  as  mechanical  mixture, 
solution,  alloy,  etc.  At  one  time  it  was  thought  that  no 
strict  line  of  division  could  be  drawn  between  these  two 
classes.  The  same  ultimate  causes  were  supposed  to 
give  rise  to  the  formation  of  both  ;  and  the  constituents 
of  both  were  supposed  to  be  held  together  by  the  same 
agent.  It  may  be  shown  that  there  is  a  marked  difference 
between  them,  sufficient  to  enable  us  to  say,  in  most 
cases,  with  which  we  have  to  deal. 

Firstly,  if  we  examine  chemical  compounds,  we  find  that 
one  of  their  most  prominent  characteristics  is  the  posses- 
sion of  properties  which  differ  entirely  from  those  of  their 
constituents.  Hydrogen,  an  inflammable  gas,  and  oxygen, 
a  gas  and  energetic  supporter  of  combustion,  combine  to 
form  a  liquid,  water,  which  is  not  inflammable  and  does 
not  support  combustion.  Hydrochloric  acid,  a  gas  that 
turns  vegetable  blues  red,  and  ammonia,  a  gas  that  turns 
vegetable  reds  blue,  unite  to  form  sal-ammoniac — a  solid 
that  is  without  influence  upon  vegetable  colors.  Chlorine, 
a  gas,  and  mercury,  a  liquid,  give  a  solid  with  none  of 
the  properties  of  either.  The  number  of  these  examples 
might  be  increased  indefinitely,  and  in  each  case  a  similar 
result  would  be  reached. 

Secondly,  it  will  be  found  that  no  purely  mechanical 
means  will  suffice  to  separate  the  constituents  of  a 
chemical  compound  from  each  other ;  but  for  this  pur- 
pose one  of  the  so-called  physical  forces,  as  heat,  light, 
electricity,  chemism,  is  necessary. 

Thirdly,  the  most  important  characteristic  of  chemical 
compounds  is  to  be  found  in  the  proportion  by  weight  in 
which  the  constituents  are  bound  together.  As  regards 
any  compound  of  two  elements,  it  is  a  fact  that  the  con- 
stituents are  present  in  fixed  proportions  by  weight.  If 
we  bring  these  elements  together  without  reference  to 
their  quantities,  and  the  proper  conditions  be  brought 
about  to  induce  combination,  it  is  found  that  a  definite 
quantity  of  one  combines  with  a  definite  quantit}7"  of  the 


ATOMIC    THEORY  —  ATOMIC    WEIGHTS,    ETC.         29 

other;  and,  if  the  quantity  of  either  present  is  in  excess 
of  the  fixed  quantity  necessary  for  the  formation  of  the 
compound,  this  excess  will  remain  in  its  original  form 
after  combination  has  taken  place.  We  can  only  vary 
the  proportions  to  a  very  limited  extent,  and  then  not 
gradually,  but  according  to  a  fixed  rule.  This  is  the 
circumstance  which  above  all  others  enables  us  to  assert 
positively  that  a  given  body  is  or  is  not  a  chemical  com- 
pound. 

Mechanical  Mixtures. — To  compare  the  second  class 
of  compounds  with  chemical  compounds  proper,  let  us 
first  take  the  so-called  mechanical  mixtures.  If  we  bring 
oxygen  and  nitrogen  together,  a  homogeneous  mixture  of 
the  two  is  formed,  and  this  possesses  the  properties  of  both 
oxygen  and  nitrogen  ;  such  a  mixture,  for  instance,  is  the 
atmosphere  of  the  earth.  Gases  mix  in  this  way  by  virtue 
of  their  inherent  tendency  to  expand  indefinitely  and 
completely  fill  the  space  offered  to  them.  It  is  hence 
unnecessary  to  suppose  that  any  special  force  acts  in  this 
gas  to  hold  the  constituents  together.  Many  solids  may 
be  mixed  in  various  ways,  but  no  matter  how.  finely  we 
may  divide  them,  nor  how  intimately  we  may  mix  them, 
provided  chemical  combination  does  not  take  place,  we 
can  again  separate  the  constituents  of  the  mixture  by 
mechanical  means ;  and  the  mixture  possesses  all  the 
original  properties  of  its  constituents.  In  both  these 
cases,  further,  the  most  varied  quantities  of  the  sub- 
stances may  be  employed,  and,  under  the  same  conditions, 
the  mixtures  will  be  formed  just  as  readily  with  one  pro- 
portion as  with  another. 

Solutions  and  Alloys. — On  the  other  hand,  those  com- 
pounds which  are  known  under  the  names  of  solutions 
and  alloys  are  more  closely  allied  to  chemical  compounds. 
We  may  have  gases,  liquids,  or  solids  in  the  state  of 
solution,  that  is,  in  combination  with  some  liquid  body, 
and  to  all  appearance  themselves  in  the  liquid  form.  The 
external  properties  of  one  of  the  constituents  are  no 
longer  recognizable,  and  they  are,  indeed,  in  part  lost. 
A  gas  loses  its  ordinary  elasticity  when  dissolved  in  a 
liquid.  A  solid  loses  the  cohesion  which  before  held  its 
particles  together.  Two  liquids  combined  in  this  way 

3* 


30          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

lose  some  of  their  original  properties,  and  receive  new 
ones  that  represent  a  mean  between  the  lost  ones.  In  all 
these  instances  some  force  must  be  imagined  as  acting 
between  the  particles  of  the  dissolved  bodies  and  the 
particles  of  the  solvents,  which  is  greater  in  its  effect 
than  the  cohesion  that  originally  held  together  the  par- 
ticles of  the  solid  or  liquid,  or  the  repulsion  that  was 
exerted  between  the  particles  of  the  gas.  Further,  we 
have  the  case  of  alloys  or  compounds  of  two  or  more 
metals.  These  alloys  present  all  the  appearance  of  per- 
fectly homogeneous  bodies,  but  nevertheless  possess  most 
of  the  properties  of  the  constituents.  Here,  too,  some 
force  must  be  considered  as  acting  between  the  unlike 
particles  which  differs  from  the  ordinary  force  of  cohesion. 

On  examining  the  above-mentioned  cases  more  care- 
fully, we  find  that  there  is,  in  almost  all  cases,  a  limit  to 
the  action  of  the  force.  Substances  that  are  soluble  in 
water  are  not  usually  soluble  to  an  unlimited  extent ;  on 
the  contrary,  for  any  given  temperature,  the  proportion 
of  the  substance  that  can  be  dissolved  is  fixed.  But, 
between  this  fixed  amount  and  the  smallest  possible 
quantity  of  the  substance,  all  proportions  are  equally 
well  dissolved.  Some  liquids  mix  with  each  other  in  all 
proportions,  a  perfectly  homogeneous  liquid  being  the 
result.  Others  dissolve  each  other  to  only  a  limited 
extent,  the  limits  being,  as  in  the  case  of  solids  and 
liquids,  fixed  for  any  given  temperature. 

Whatever  the  force  may  be  that  is  supposed  to  be  the 
essential  agent  in  the  formation  of  these  compounds  in 
variable  proportions,  it  is  certain  that  the  law  or  laws  of 
its  action  have  not  been  discovered  up  to  the  present. 
Some  have  looked  upon  it  as  identical  with  chemism.  yet 
it  appears  that  very  distinct  differences  between  the  two 
can  be  pointed  out. 

The  first  feature  of  these  compounds  that  indicates  a 
radical  difference  in  the  two  forces  is  the  retaining  of  the 
chief  original  properties  of  the  constituents.  If  we  dis- 
solve sodium  chloride  in  water,  we  can  obtain  all  the 
important  effects  from  the  solution  that  we  could  from 
the  solid  substance,  and  added  to  these  we  would  then 
further  obtain  the  effects  of  the  water.  And  the  same 
holds  good  for  all  solutions  ;  they  can  produce  the  effects 
of  the  substances  dissolved  and  of  the  solvent  combined. 


ATOMIC    THEORY. ATOMIC    WEIGHTS,    ETC.         31 

As  we  have  seen,  tliis  is  not  true  of  chemical  compounds 
proper. 

Again,  and  most  especially  is  the  difference  marked,  if 
we  consider  the  proportions  by  weight  in  which  the  sub- 
stances combine  in  the  two  cases.  Whereas,  whenever 
chemical  compounds  are  formed,  the  constituents  com- 
bine in  fixed  proportions, — in  the  case  of  mixtures,  solu- 
tions, alloys,  the  constituents  may  combine  in  all  possible 
proportions  up  to  a  certain  fixed  limit. 

Whether  it  would  be  expedient  then  to  consider  chem- 
ism  and  the  force  that  is  the  cause  of  the  formation  of 
solutions,  etc.,  as  identical,  but  differing  in. degree,  is  a 
question  that  cannot  be  here  discussed.  Certain  it  is, 
from  the  above  remarks,  that  there  exists  sufficient  differ- 
ence between  them  to  warrant  us  for  the  present  in  restrict- 
ing the  use  of  the  name  chemism  to  the  designation  of 
that  force  which  is  the  essential  cause  of  the  formation 
of  chemical  compounds.  In  this  sense  it  will  be  used  in 
the  following  pages.  The  atomic  theoi1}*  accounts  for  the 
fact  that  bodies  combine  in  definite  proportions  by  sup- 
posing them  to  combine  atom  to  atom,  and  these  atoms 
to  possess  definite  weights.  According  to  this,  chemism, 
in  its  restricted  sense,  is  the  force  which  is  exerted  between 
atoms.  It  will  be  shown  that  these  atoms  ma}'  be  either 
like  or  unlike.  If  they  are  like,  the  resulting  body  is  an 
element ;  if  they  are  unlike,  the  resulting  body  is  a  chem- 
ical compound. 


It  is  plain,  from  the  foregoing,  that  chemical  com- 
pounds and  elements  are  the  only  substances  the  study 
of  which  can  lead  to  definite  conclusions  concerning  the 
action  of  chemism,  and  hence  we  must  confine  ourselves 
to  these  in  our  subsequent  study  of  this  force.  And  first, 
we  must  return  to  that  fundamental  problem  of  chemistry 
— the  determination  of  atomic  weights.  It  having  been 
shown  that  results  reached  by  the  methods  already  given 
must  necessarily  be  uncertain,  we  now  proceed  to  attack 
the  subject  from  a  wholly  new  side. 


II. 

EXAMINATION  OF  GASEOUS  ELEMENTS  AND 
COMPOUNDS. 

As  bodies  present  themselves  to  us  in  three  different 
states  of  aggregation,  the  solid,  liquid,  and  gaseous, 'so 
our  methods  of  investigation  of  bodies  must  take  differ- 
ent directions.  The  gases  possess  certain  properties 
that  are  not  possessed  by  solids  and  liquids,  and  in  solids 
and  liquids  we  detect  certain  general  properties  that  we 
cannot  detect  in  gases.  The  study  of  bodies  in  the  form 
of  gas  or  vapor  has  led  to  very  important  results  of  last- 
ing influence  upon  the  science,  and  to  these  let  us  direct 
our  attention. 

Investigations  of  Gay  Lussac. — In  the  year  1808,  Gay 
Lussac  and  Humboldt  discovered  the  fact  that  when 
hydrogen  and  oxygen  combined  to  form  water,  the  com- 
bination takes  place  between  2  volumes  of  hydrogen  and 
1  volume  of  oxygen.  The  simplicity  of  this  relation 
induced  Gay  Lussac  to  take  up  the  study  of  other  gaseous 
bodies,  with  the  view  of  determining  whether  similar 
relations  existed  between  the  volumes  of  other  combining 
gases.  His  researches  permitted  him  soon  after  to  deduce 
the  following  law  of  combination  by  volumes : — 

When  two  or  more  gaseous  constituents  combine 
to  form  a  gaseous  compound,  the  volumes*  of  the 
individual  constituents  as  well  as  their  sum  bear  a 
simple  relation  to  the  value  of  the  compound. 
Thus,  when  hydrogen  and  chlorine  unite  to  form  hydro- 
chloric acid,  it  was  found  that  1  volume  of  hydrogen  and 
1  volume  of  chlorine  formed  2  volumes  of  hydrochloric 

*  In  all  cases  where  the  volumes  of  different  gases  are  com- 
pared, the  gases  are,  of  course,  supposed  to  be  under  the  same 
conditions  of  pressure  and  temperature. 


GASEOUS  ELEMENTS  AND  COMPOUNDS.      33 

acid  gas.  Two  volumes  of  hydrogen  and  1  volume  of 
oxygen  gave  2  volumes  of  water-vapor;  2  volumes  of 
nitrogen  and  1  volume  of  oxygen  gave  2  volumes  of 
nitrous  oxide.  Further,  3  volumes  of  hydrogen  and  1 
volume  of  nitrogen  gave  2  volumes  of  ammonia,  etc.  etc. 

On  comparing  this  result  with  that  already  obtained 
by  Palton,  and  making  use  of  the  atomic  theory,  accord- 
ing to  which  combination  between  elements  takes  place 
between  their  atoms,  we  see  that  some  simple  relation 
exists  between  the  volumes  of  gases  and  the  relative 
number  of  atoms  contained  in  these  volumes.  This  we 
may  express  in  general  terms  as  follows : — 

The  number  of  atoms  contained  in  a  given  volume  of 
a  gaseous  body  forms  a  simple  ratio  with  the  number  of 
atoms  contained  in  the  same  volume  of  other  gaseous 
bodies. 

As  will  be  readily  seen,  this  gives  no  foundation  for 
the  determination  of  atomic  weights,  inasmuch  as  we 
have  no  means  of  fixing  the  va-lue  of  the  "  simple  ratio," 
and  without  this  we  cannot  determine  the  relative  number 
of  atoms  contained  in  a  given  volume  of  gas.  We  know 
that  2  volumes  of  hydrogen  combine  with  1  volume  of 
oxygen,  and  we  know  that  2  parts  by  weight  of  hydrogen 
combine  with  16  parts  by  weight  of  oxygen.  Further, 
the  atomic  theory  tells  us  that  a  certain  number  of  atoms 
of  hydrogen  of  fixed  weight  combine  with  a  certain  number 
of  atoms  of  oxygen  of  fixed  weight,  and  that  these  num- 
bers bear  a  simple  relation  to  each  other;  hence  the  rela- 
tion between  the  number  of  atoms  of  hydrogen  in  the  2 
volumes,  and  the  number  of  atoms  of  oxygen  in  the  1 
volume,  must  be  a  simple  one,  but  the  facts  do  not  fur- 
nish us  with  sufficient  data  to  enable  us  to  state  what 
this  relation  is  ;  without  further  aid  either  from  new  facts 
or  speculations,  we  cannot  say  what  the  atomic  weights 
of  these  elements  are. 

Auogadro's  Speculations. — The  numbers  expressing 
the  specific  gravities  of  gases  or  vapors  are  those  numbers 
which  express  the  relative  weights  of  like  volumes  of 
these  gases  or  vapors.  Hence  it  is  but  restating,  in 
another  form,  the  principle  above  laid  down  to  say  that 
the  specific  gravities  of  gaseous  bodies  bear  a  simple 
relation  to  the  atomic  weights  of  these  bodies.  The  force 


34 


DISCUSSION    OP    ATOMS    AND    MOLECULES. 


of  this  statement  will  be  readily  recognized  on  comparing 
the  specific  gravities  of  some  gases  with  the  atomic 
weights  of  the  same  bodies  determined  by  chemical  means. 
The  atomic  weights  as  determined  by  chemical  means, 
however,  differed  from  each  other  according  to  the  method 
employed  in  this  determination;  but  the  difference  being 
that  between  one  number  and  some  multiple  of  that 
number,  it  is  immaterial  which  of  these  numbers  we 
employ  for  the  purposes  of  the  comparison.  Let  us  then 
take  the  first  of  those  determined.  The  following  table 
hardly  needs  explanation.  The  numbers  in  the  second 
column  (d)  represent  the  specific  gravities  of  the  elements 
in  the  form  of  gas  or  vapor;  the  fourth  column  contains 

the  ratios  between  the  atomic  weights  (^4)  and  d  =   ,• 


Element. 

d. 

A. 

A 

d    ' 

Hydrogen 

0.0692 

I 

14.45 

Chlorine 

2.440 

35,5 

14.55 

Bromine 

5.54 

80 

14.44 

Iodine 

8.716 

127 

14.57 

Oxygen 

1.10563 

8 

7.24 

Sulphur 

2.23 

16 

7.17 

Selenium 

5.68 

39.7 

6.99 

Tellurium 

9.08 

64 

7.05 

Nitrogen 

0.9713 

14 

14.41 

Phosphoru 

3 

4.50 

31 

6.89 

Arsenic 

10.6 

75 

7.08 

Mercury 

7.03 

100 

14.22 

Cadmium 

3.94 

56 

14.21 

We  see  thus  that  the  relation  between  the  specific 
gravity  and  the  atomic  weight  of  seven  of  these  thirteen 
elements  is  the  same,  being  expressed  by  a  number  vary- 
ing but  little  from  14.4.  Jn  the  six  remaining  elements 
of  the  list  also  the  relation  is  virtually  the  same,  about 
7.1.  And,  in  the  latter  case,  the  ratio  is  expressed  by  a 
number  half  as  great  as  the  first. 

A  consideration  of  these  relations  led  Avogadro,*  in 
1811,  to  propose  an  hypothesis  which,  if  it  could  be  well 


*  In  1814  Ampere  proposed  a  similar  hypothesis. 


GASEOUS    ELEMENTS    AND    COMPOUNDS.  35 

founded,  would  prove  of  the  greatest  service  in  simplify- 
ing the  problem  of  determining  the  atomic  weights — at 
least  of  gaseous  bodies.  It  will  be  seen  that,  if  in  the 
above  schedule  the  atomic  weights  of  oxygen,  sulphur, 
selenium,  tellurium,  phosphorus,  and  arsenic  be  doubled, 

A 

the  ratio  —  for  all  the  elements  in  the  list  will  be  the 
d 

same  constant  number,  viz.,  about  14.4.  But  the  atomic 
weights  above  given  have  been  determined  purely  empiri- 
cally, and  we  are  as  much  justified  in  considering  these 
numbers  doubled  the  true  atomic  weights,  as  we  are  in 
accepting  the  ones  given.  If  we  make  this  change,  then 
for  the  above  thirteen  elements,  the  following  statement 
would  be  true :  The  atomic  weights  are  to  each  other  as 
the  specific  gravities  of  the  vapors.  An  examination  of 
compound  gaseous  bodies  showed  further  that  a  simple 
relation  also  existed  between  their  specific  gravities  and 
the  numbers  expressing  the  sum  of  the  atomic  weights  of 
the  constituents,  these  sums  being  to  each  other  as  the 
specific  gravities.  Avogadro's  hypothesis  to  account  for 
these  relations  may  be  stated  in  the  following  words : — 

All  gases  or  vapors,  without  exception,  contain, 
in  the  same  volume,  the  same  number  of  ultimate 
particles  or  molecules. 

The  molecules  were  not  considered  to  be  identical  with 
the  atoms,  and  it  is  well  here  to  draw  the  distinction 
between  the  two  as  clearly  as  possible.  Molecules  of 
compounds,  as  understood  by  Avogadro,  and  as  under- 
stood at  present,  are  the  theoretically  smallest  particles 
of  these  compounds.  The  molecule  of  water  is  the 
smallest  particle  of  water  that  can  exist  as  water.  As 
water,  however,  is  composed  of  two  elements,  of  course 
the  smallest  particle  of  water  must  necessarily  still  be 
divisible  into  these  constituents.  The  component  parts 
of  molecules  are  called  atoms,  and  these  are  indivisible. 
In  the  case  of  water,  the  molecule  has  the  same  composi- 
tion as  the  mass  of  the  compound,  but,  as  will  be  shown, 
this  molecule  of  water  consists  of  two  atoms  of  hydrogen 
and  one  atom  of  oxygen.  The  atoms  are  held  together 
by  chemism,  the  molecules  by  cohesion. 

Now  there  are  good  reasons,  which  will  be  considered 
below,  for  believing  that,  in  their  internal  structure, 
elementary  substances  are,  in  some  respects,  analogous 


36          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

to  compounds,  and  this  belief  was  made  a  fundamental 
condition  of  Avogadro's  hypothesis.  According  to  this, 
it  is  impossible  by  purely  mechanical  means  to  subdivide 
an  element  so  far  as  to  reach  its  atoms ;  but  if  we  suppose 
it  divided  as  far  as  possible  by  such  means,  we  reach,  as 
in  the  case  of  compounds,  the  molecule  of  the  element, 
which  is  the  smallest  particle  of  the  element  that  can 
exist  and  play  the  part  of  the  element.  This  molecule, 
however,  usually  consists  of  atoms  which  are  held 
together  by  chemism,  and  can  hence  only  be  separated 
by  some  means,  other  than  mechanical,  that  are  known 
to  have  the  power  of  overcoming  the  force. 

From  these  considerations  we  are  enabled  to  give 
definitions  of  the  terms  atom  and  molecule : — 

A  molecule  is  the  smallest  particle  of  a  compound 
or  element  that  is  capable  of  existence  in  a  free 
state.  A  breaking  up  of  the  molecule  necessitates 
the  destruction  of  the  properties  of  the  compound, 
and  almost  always  of  those  of  the  element. 

Atoms  are  the  indivisible  constituents  of  mole- 
cules. They  are  the  smallest  particles  of  elements 
that  can  take  part  in  chemical  reactions,  and  are, 
for  the  greater  part,  incapable  of  existence  in  the 
free  state,  but  are  always  found  in  combination  with 
other  atoms,  either  of  the  same  kind  or  of  different 
.  kinds. 

And  now  the  justice  of  the  definitions  of  elements  and 
compounds  given  above  (see  pp.  27, 28)  will  be  recognized, 
viz.,  an  element  is  a  substance  made  up  of  atoms  of  the 
same  kind  ;  a  compound  is  a  substance  made  up  of  atoms 
of  unlike  kind. 

Recognizing  thus  fully  the  distinction  between  atoms 
and  molecules,  we  are  prepared  to  further  follow  the 
reasoning  of  Avogadro. 

The  experiments  of  Gay  Lussac  had  already  proved 
that,  under  the  influence  of  heat,  all  gases  expand  in  the 
same  proportion  for  the  same  increase  of  temperature, 
and  diminished  in  volume  to  the  same  extent  for  the 
same  decrease  of  temperature.  Further,  Mariotte,  in 
France,  and  Boyle,  in  England,  had  shown  that  all  gases 
conducted  themselves  in  the  same  way  under  the  influ- 
ence of  increased  or  decreased  pressure ;  that  for  the 
same  increase  or  decrease  of  pressure  the  consequent 


GASEOUS    ELEMENTS    AND    COMPOUNDS.  37 

decrease  or  increase  of  volume  was  for  the  same  volume 
of  all  gases  the  same.  These  facts  considered  indepen- 
dently would  lead  to  a  suspicion  that  all  gases  possess  a 
similar  internal  structure,  and  the  simplest  hypothesis  to 
account  for  this  is  just  the  hypothesis  of  Avogadro — that 
the  same  volumes  of  all  gaseous  bodies  contain  the  same 
number  of  molecules.  This  subject  has  been  considered 
exhaustively  from  a  purely  physical  standpoint.  The 
principles  of  the  mechanical  theory  of  gases  being  ac- 
cepted, it  was  shown  that  the  hypothesis  of  Avogadro 
would  logically  follow ;  and  then,  by  a  purely  mathemati- 
cal process  of  reasoning,  it  was  shown  that  the  hypothesis 
was  an  absolute  necessit}^.  A  discussion  of  the  subject 
in  the  direction  indicated  cannot  here  be  taken  up.  For 
those  who  desire  to  follow  the  discussion,  and  to  become 
acquainted  with  the  methods  that  hjave  led  to  the  result 
mentioned,  the  following  references  will  be  of  service : 
Kronig,  Poggendorff's  Annalen,  99,  316  ;  Clausius,  Fogg. 
Ann.,  100,  360;  Naumann,  Annalen  der  Chemie  u.  Phar- 
macia, 1870,  Suppl.  Band  7,  340  ;  Pfaundler,  Pogg.  Ann., 
144,  428;  Maxwell,  Phil.  Mag.,  1860,  [4]  19,19;  Phil. 
Trans.,  1867,  1 ;  Phil.  Mag.,  1868,  [4]  35,  185  ;  Thomsen, 
Berichte  der  deutsch.  chem.  Gesellschaft  zu  Berlin,  1870, 
829;  Lothar  Meyer,  ibid.,  1870,  864;  Thomsen,  ibid., 

1870,  954;  Lothar  Meyer,  ibid.,  1871,  28;   Mees,  ibid., 

1871,  272. 

As  a  grand  result  of  the  investigations  that  have  been 
made  on  the  internal  structure  of  gases,  it  may  be  stated 
that  Avogadro's  hypothesis  has  throughout  asserted  its 
correctness,  and  it  has  at  last  become  of  fundamental 
importance  in  the  science  of  chemistry.  It  is  at  present 
almost  universally  accepted  by  chemists,  some,  indeed, 
going  so  far  as  to  speak  of  it  as  a  law. 

Determination  of  Molecular  Weights. — What,  then, 
do  we  gain  by  accepting  the  hypothesis  ?  It  is  plain 
that  if  equal  volumes  of  all  gases  contain  the  same  num- 
ber of  molecules,  we  have  a  means  given  us  at  once  for 
ascertaining  the  relative  weights  of  these  molecules.  We 
have  merely  to  determine  the  relative  weights  of  equal 
volumes  of  the  gases,  and  the  numbers  obtained  will  bear 
the  same  relations  to  each  other  as  the  molecular  weights. 
Then  accepting  the  weight  of  some  molecule  as  a  standard, 
4 


38          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

and  expressing  the  weights  of  the  others  in  terms  of  this 
standard,  the  molecular  weights  are  determined.  If  we 
call  the  molecular  weight  of  hydrogen  2,  for  instance, 
and  find  the  relation  between  this  number  and  the  number 
expressing  the  specific  gravity  of  hydrogen,  then  we  have 
also  found  the  number  expressing  the  relation  between 
the  molecular  weights  and  specific  gravities  of  all  gases 
and  vapors,  without  exception.  The  specific  gravity  of 
hydrogen  as  compared  with  air  is  0.06926,  the  ratio 

2 
rTnT-"^—  28.88  ;  hence,  the  specific  gravity  multiplied  by 

28.88  gives  the  molecular  weight.  If  d  =  specific  gravity, 
M=  molecular  weight,  then  the  following  formula  will 
express  the  relation  : — 

M  =  d  x  28.88. 

As  the  molecule  of  a  body  consists  of  atoms,  so  the 
molecular  weight  must  be  the  sum  of  the  weights  of  those 
atoms.  In  the  case  of  an  element,  the  atoms  being  of  the 
same  kind,  the  molecular  weight  must  be  a  multiple  of 
the  atomic  weight.  Now  we  have  already  seen  that  for 
every  element  we  can,  by  chemical  analysis,  determine 
some  number  that  must  represent  either  the  atomic  weight 
itself  or  a  multiple  of  this  weight.  Hence,  we  have  it  in 
our  power  to  determine  by  chemical  analysis  a  multiple 
of  the  molecular  weight  of  an  element  or  a  compound. 
This  determination  can  be  made  with  greater  accuracy 
than  that  of  the  specific  gravity  of  a  gas  or  vapor,  and 
it  must  be  employed  as  a  control  for  the  determination 
of  molecular  weights  b}7  Avogadro's  rule.  If  we  take 
water,  for  instance,  we  find  by  means  of  anatysis  that  its 
molecular  weight  is  either  9  or  some  simple  multiple  of 
9.  The  specific  gravity  of  water-vapor  is  0.623,  which 
multiplied  by  28.88  gives  1*7.99  as  the  molecular  weight. 
Hence,  we  conclude  that  18  is  the  true  molecular  weight 
of  water.  The  coincidence  of  the  numbers  determined 
according  to  the  two  methods  will  be  seen  in  the  case  of 
a  few  elements  and  compounds  in  the  following  table. 
The  numbers  under  M  are  those  found  by  the  analytical 
method,  that  one  of  a  series  of  multiples  being  selected 
that  agrees  most  nearly  with  the  number  found  according 
to  the  rule  ^/=28.88'X  d. 


GASEOUS    ELEMENTS    AND    COMPOUNDS. 


39 


Name. 

Specific  gr.  =  d. 

28.88  X^ 

M. 

Hydrogen  .... 

0.06926      [        2 

2 

Nitrogen     .... 

0.9713 

28.05 

28 

Oxygen       .... 

1.1  0503 

31.93 

32 

Sulphur      .... 

2.23 

64.4 

64 

Chlorine     .... 

2.45 

70.75 

71 

Cadmium   .... 

3.94 

113.78 

112 

Phosphorus 

4.35 

125.62 

124 

Bromine     .... 

5.54 

159.99 

160 

Selenium    .... 

5.68 

164.03 

158.8 

Mercury     .... 

6.98 

201.58 

200 

Water         .... 

0.623 

17.99 

18 

Hydrochloric  acid 

1.247 

36.11 

36.5 

Sulphur  dioxide         .         ; 

2.247 

64.89 

64 

Ammonia  .         .         .         . 

0.597 

17.24 

17 

Phosphorus  trichloride 

4.88 

140.93 

137.5 

Arsenic  trichloride     .         .-.* 

6.30- 

181.94 

181.5 

Boron  chloride  . 

3.942 

113.84 

117.5 

Marsh  -gas 

0.557 

1608 

16 

Methyl  chloride 

1.736 

50.13 

50.5 

Chloroform 

4.20 

121.29 

119.5 

Tin  chloride 

9.20 

265.69 

260 

Silicon  chloride 

5.94 

171.55 

170 

Zinc-methyl 

3.29 

95.02 

95.2 

Aluminium  chloride 

9.35 

270.03 

267 

Iron  trichloride 

11.39 

328.94 

325 

Number  of  Atoms  in  the  Molecules  of  Elements. — 
Although  we  are  thus  enabled  by  a  simple  process  to 
determine  the  molecular  weights  of  some  of  the  elements, 
an  important  part  of  the  real  problem — the  determination 
of  the  atomic  weights — remains  yet  to  be  solved.  If  we 
could  know  in  each  case  how  many  atoms  are  contained 
in  a  molecule,  our  difficulties  would  be  at  an  end,  but 
this  we  plainly  do  not  know  without  the  introduc- 
tion of  considerations  of  a  different  kind  from  those 
with  which  we  have  had  to  deal  as  yet.  Avogadro 
reasoned  as  follows,  with  reference  to  some  of  the  simple 
chemical  compounds:  Given  hydrochloric  acid,  it  is  re- 
quired to  know  how  many  atoms  are  contained  in  a 
molecule  of  hydrogen  and  in  a  molecule  of  chlorine.  If 
in  a  certain  volume  of  hydrogen  there  are  contained  say 
100  molecules,  then  in  the  same  volume  of  chlorine  there 
is  contained  the  same  number  of  molecules.  Now  it  is 


40          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

known  that  1  volume  of  hydrogen  combines  with  1 
volume  of  chlorine.  Two  volumes  of  hydrochloric  acid 
gas  are  formed,  and,  according  to  the  hypothesis,  these 
two  volumes  in  the  case  under  consideration  contain  200 
molecules.  But  each  molecule  of  hydrochloric  acid  must 
contain  at  least  one  atom  of  hydrogen  and  one  atom  of 
chlorine;  hence,  in  100  molecules  of  hydrogen  and  100 
molecules  of  chlorine  there  must  be  contained  at  least 
200  atoms  of  chlorine  and  200  atoms  of  hydrogen,  or  a 
molecule  of  either  hydrogen  or  chlorine  must  contain  at 
least  two  atoms  of  the  corresponding  element.  Further,  as 
no  simpler  compound  than  hydrochloric  acid  of  hydrogen 
nor  of  chlorine  is  known,  any  conclusions  which  we  may 
draw  from  a  consideration  of  this  compound  must  be  valid 
for  all  compounds  of  these  elements.  The  supposition  that 
two  atoms  form  the  molecule  of  hydrogen  and  of  chlo- 
rine satisfies  all  the  facts  known  to  us,  and  we  hence  rest 
with  this  supposition.  As  we  take  the  atomic  weight  of 
hydrogen  as  the  unit  of  these  weights,  its  molecular 
weight  will  then  necessarily  be  2,  on  this  basis ;  and  for 
this  reason  the  number  2  was  taken  in  the  above  table  as 
the  standard  of  comparison  for  other  molecular  weights. 
It  must,  however,  be  distinctly  borne  in  mind  that  no 
proof  is  here  given  of  the  absolute  number  of  atoms 
contained  in  the  molecules  of  hydrogen  and  chlorine. 
We  cau  only  say  that  at  least  2  atoms  must  be  present  in 
each  of  the  molecules.  There  may  be  a  much  greater 
number,  but  the  data  permit  no  speculations  beyond  this 
number  2. 

For  all  similar  cases  a  similar  process  of  reasoning 
may  be  employed,  and  with  the  same  results.  Whenever 
1  volume  of  an  elementary  gas  or  vapor  combines  with  1 
volume  of  another  elementary  gas  or  vapor  to  form  2 
volumes  of  a  compound  gas  or  vapor,  we  are  justified  in 
concluding  that  each  molecule  of  these  elements  contains 
two  atoms.  The  elements  that  come  under  this  head  are 
hydrogen,  chlorine,  bromine,  and  iodine. 

If  we  pass  to  oxygen,  we  find  a  material  difference  in 
the  method  of  combination.  Here  2  volumes  of  hydrogen 
combine  with  1  volume  of  oxygen  to  form  2  volumes  of 
water-vapor.  Let  us  reason  as  above.  If  in  1  volume 
of  oxygen  there  are  contained  100  molecules,  then  in  2 
volumes  of  hydrogen  there  are  200  molecules.  These 


GASEOUS    ELEMENTS    AND    COMPOUNDS.  41 

300  molecules  combine  to  form  200  molecules  of  the 
compound.  Now,  in  the  molecule  of  water,  there  must 
be  contained  at  least  one  atom  of  oxygen  and  one 
atom  of  hydrogen ;  hence,  there  must  be  at  least  200 
atoms  of  oxygen  and  200  atoms  of  hydrogen.  But  we 
know  that  in  the  original  200  molecules  of  hydrogen 
there  were  contained  400  atoms;  hence,  in  each  molecule 
of  water,  there  must  be  2  atoms  of  hydrogen.  Water  is 
the  simplest  compound  of  oxygen  known  to  us  (i.e.,  it 
contains  the  smallest  quantity  of  oxygen  in  the  molecule), 
and  on  this  account  we  suppose  the  molecule  of  water  to 
contain  1  atom  of  oxygen.  If,  then,  each  water  molecule 
contains  2  atoms  of  hydrogen  and  1  atom  of  oxygen,  in 
the  200  molecules  of  water  there  are  200  atoms  of  oxygen 
and  400  atoms  of  hydrogen,  and  these  are  obtained  from 
100  molecules  of  oxj'gen  and  200  molecules  of  hydrogen. 
Therefore,  each  molecule  of  oxygen,  as  well  as  each 
molecule  of  hydrogen  contains  2  atoms.  For  sulphur 
the  same  is  true,  and  is  proved  in  a  similar  manner. 

Another  method  of  reasoning,  starting  from  entirely 
different  facts,  also  led  Favre  and  Silbermann  to  suggest 
that  the  molecule  of  oxygen  consists  of  two  atoms.  They 
proved  that  carbon,  when  burned  in  protoxide  of  nitrogen, 
evolves  more  heat  than  when  burned  in  oxygen.  The 
most  natural  interpretation  of  this  fact  consists  in  admit- 
ting that,  in  each  experiment,  a  chemical  combination  is 
destroyed  whilst  another  is  formed ;  and  that  the  amount 
of  heat  actually  evolved  is  the  difference  between  the 
amount  of  heat  disengaged  by  the  union  of  carbon  with 
oxygen  and  the  amount  of  heat  absorbed  by  the  decom- 
position of  tUe  oxide  of  oxygen  in  the  first  instance,  and 
of  oxide  of  nitrogen  in  the  second.  And,  if  the  thermic 
effect  is  less  with  oxygen  than  with  protoxide  of  nitrogen, 
that  is  due  to  the  circumstance  that  oxide  of  oxygen  (the 
molecule  of  oxygen  0=0)  absorbs  more  heat  in  decom- 
posing than  does  the  molecule  of  protoxide  of  nitrogen. 

One  volume  of  nitrogen  combines  with  3  volumes  of 
hydrogen  to  form  2  volumes  of  ammonia.  Hence  in  the 
molecule  of  ammonia  there  are  3  atoms  of  hydrogen,  and, 
ammonia  being  the  simplest  compound  of  nitrogen,  we 
suppose  that  these  3  atoms  of  hydrogen  are  combined 
with  1  atom  of  nitrogen.  As  each  molecule  of  ammonia 
contains  one  atom  of  nitrogen,  and  as,  further,  there  are 

4* 


42          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

formed  twice  as  many  molecules  of  ammonia  as  there 
were  molecules  of  nitrogen  originally,  it  follows  that  the 
molecule  of  nitrogen  contains  at  least  2  atoms. 

By  this  means  we  are  enabled  to  determine  the  atomic 
weight  of  the  elements  mentioned,  for,  if  in  their  mole- 
cules 2  atoms  are  contained,  we  have  only  to  divide  the 
molecular  weight — found  by  Avogadro's  rule,  and  cor- 
rected by  analytical  methods — by  two.  But,  accepting 
the  atomic  weights  of  hydrogen,  chlorine,  bromine,  and 
iodine  as  known,  we  are  enabled  by  another  process  to 
determine  the  atomic  weights  of  such  elements  as  com- 
bine with  these  to  form  gaseous  compounds. 

Take  again  water.  We  find  by  a  comparison  of  the 
compounds  of  oxygen  that  the  molecule  of  water,  as 
stated  above,  contains  the  smallest  quantity  of  this 
element ;  and  hence  we  suppose  this  quantity  to  repre- 
sent 1  atom.  We  first  find  the  molecular  weight  from  the 
specific  gravity  of  the  vapor.  This  is  18.  We  analyze 
the  compound,  and  find  that  it  contains  88.89  per  cent, 
oxygen  and  11.11  per  cent,  hydrogen,  or  8  pails  of  oxy- 
gen to  1  part  of  hydrogen.  This  being  the  relative  pro- 
portion of  the  two  elements  in  the  compound,  in  18  parts 
by  weight,  which  represent  the  molecule,  there  are  con- 
tained 16  parts  of  oxygen  and  2  parts  of  hydrogen.  The 
atomic  weight  of  oxygen  is  hence  16,  and  in  water  one 
atom  of  ox^ygen  is  combined  with  2  atoms  of  hydrogen. 
In  the  case  of  nitrogen,  the  application  of  the  same 
principle  must  also  lead  to  the  same  number  previously 
found,  viz.,  14.  We  come  to  the  conclusion  that  the 
molecule  contains  one  atom  of  nitrogen.  The  molecular 
weight  of  ammonia  we  find  to  be  17.  The  analysis  shows 
us  that  the  elements  are  combined  in  the  proportion  of 
14  parts  by  weight  of  nitrogen  to  3  parts  by  weight  of 
hydrogen.  Hence  14  is  the  atomic  weight  of  nitrogen, 
and  the  molecule  of  ammonia  contains  1  atom  of  nitrogen 
with  3  atoms  of  hydrogen. 

Molecules  of  Elements  which  contain  more  or  less  than 
two  Atoms. — The  molecules  of  the  elements  considered 
contain  each  two  atoms.  This  is,  however,  not  true  of 
the  molecules  of  all  elements. 

Among  those  compounds  of  phosphorus  which  may  be 
looked  upon  as  containing  1  atom  of  this  element  in  the 


GASEOUS  ELEMENTS  AND  COMPOUNDS.      43 

molecule  is  phosphine.  The  molecular  weight  of  phos- 
phine  is  34.  The  elements  are  contained  in  it  in  the  pro- 
portion of  31  parts  of  phosphorus  to  3  parts  of  hydrogen. 
Hence  31  is  the  atomic  weight  of  phosphorus.  On  the 
other  hand,  we  find  the  molecular  weight  of  phosphorus 
itself  to  be  124,  which  shows  that  at  least  4  atoms  are 
contained  in  the  molecule.  The  same  is  true  of  arsenic. 

For  reasons  similar  to  those  given  aboATe,  the  molecule 
of  mercuric  chloride  is  supposed  to  contain  one  atom  of 
mercury.  The  molecular  weight  of  this  compound  is 
found  to  be  270.5,  and  the  elements  are  contained  in  it 
in  the  proportion  of  200  parts  of  mercury  to  71  parts  of 
chlorine,  which  gives  200  as  the  atomic  weight  of  mer- 
cury ;  and  the  atom  of  this  element  is  combined  with  2 
atoms  of  chlorine.  The  molecular  weight  of  mercury  is 
200  ;  hence  in  the  molecule  of  mercury  there  is  contained 
but  one  atom.  The  same  coincidence  of  atomic  and 
molecular  weight  is  noticed  in  connection  with  cad- 
mium.* 

*  An  interesting  experiment  has  recently  been  performed,  the 
results  of  which  also  show  that  the  molecule  of  mercury  in  all 
probability  consists  of  a  single  atom.  The  quantity  of  heat  con- 
tained in  a  gas  is  defined  as  the  total  energy  of  its  molecules,  and 
this  energy  consists  solely  in  advancing  motion,  if  the  molecule 
is  looked  upon  as  a  mere  material  point.  According  to  this,  it 
is  a  simple  matter  to  calculate  the  relation  between  the  specific 
neat  of  a  gas  at  constant  volume  and  the  specific  heat  at  constant 
pressure.  It  was  found,  however,  that,  in  the  case  of  the  gases 
examined,  the  theoretical  value  of  this  relation  was  larger  than 
the  value  actually  obtained  by  observation.  If  c  represents  the 
specific  heat  at  constant  volume,  and  c'  the  specific  heat  at  con- 
stant pressure,  then G  =  k  represents  the  relation  above  referred 

C 

to.  According  to  the  theory,  &=1.67,  whereas  observation 
gives  k  =  1.405.  In  other  words,  it  requires  more  heat  to  raise 
the  temperature  of  a  gas,  the  volume  remaining  unchanged,  than 
the  theory  demands.  The  heat  which  thus  disappears  may  be 
transformed  into  an  inter-molecular  motion  ;  i.  e.,  the  atoms 
composing  the  molecule  may  have  a  motion  relative  to  some 
centre  of  gravity.  This  motion  would  not  show  itself  as  tem- 
perature. If  the  molecule  of  the  gas  consists  of  one  atom,  then 
the  theoretical  and  observed  value  of  A;  should  be  identical.  The 
examination  of  mercury  gave  for  k  the  value  1.67,  which  is  that 
above  given  as  the  result  of  calculation.  It  is  thus  shown,  by  a 
method  entirely  independent  of  chemistry,  that  the  molecule  of 
mercury  conducts  itself  like  a  material  point,  and  this  it  could 
only  be  if  it  consisted  of  one  atom. 


44          DISCUSSION    OP    ATOMS    AND    MOLECULES. 

Varying  Number  of  Atoms  in  the  Molecule  of  one  and 
the  same  Element. — The  specific  gravity  of  the  vapor  of 
sulphur  was  stated  in  the  above  table  (p.  39)  to  be  2.23, 
and  this  led  to  the  molecular  weight  64.  Now  it  has 
been  found  that  the  specific  gravity  of  this  vapor  varies 
according  to  the  temperature  at  which  it  is  determined. 
The  determinations  which  gave  the  number  2.23  were 
made  at  temperatures  above  800°  C.  (860°  and  1040°). 
Other  determinations,  however,  made  below  800°  gave 
different  results.  At  524°  (Dumas)  and  508°  (Mitscher- 
lich)  the  specific  gravity  was  found  to  be  6.62  and  6.90 
respectively,  or  three  times  as  great  as  at  the  higher 
temperatures.  These  latter  determinations  would  give 
the  molecular  weight  1 92,  and,  if  32  be  the  atomic  weight 
of  sulphur,  then  in  the  molecule  of  the  vapor  below  800° 
there  would  be  contained  6  atoms,  whereas  above  800° 
there  are  contained  only  2  atoms  in  the  molecule. 
Selenium,  so  similar  to  sulphur  in  all  other  respects, 
presents  similar  phenomena,  though  not  in  so  marked  a 
degree.  Here,  too,  it  is  noticed  that  the  specific  gravity 
of  the  vapor  decreases  with  an  increase  of  temperature, 
or,  what,  according  to  Avogadro's  hypothesis,  is  the 
same  thing,  the  molecular  weight  decreases  with  an  in- 
crease of  temperature. 


The  application  of  the  method  thus  described  for  the 
determination  of  the  molecular  weights  of  elementary 
bodies  is  limited,  as  we  can  convert  only  a  few  of  these 
bodies  into  the  form  of  vapor.  Of  many  elements,  how- 
ever, we  know  compounds  that  are  capable  of  conversion 
into  vapor  or  are  themselves  gaseous;  and,  as  we  can 
determine  the  molecular  weights  of  these  compounds,  we 
are  in  many  cases  thus  enabled  to  determine  the  atomic 
weights.  The  following  table*  contains  a  number  of 
such  compounds,  together  with  the  densities  (d);  the  pro: 
ducts  of  the  densities  into  the  constant  28.88  (d  X  28.88) ; 
the  molecular  weights  as  found  by  analytical  methods 
(M)]  and,  finally,  the  relative  quantities  of  the  constitu- 
ents of  the  compounds  contained  in  the  molecules  as 
determined  by  analysis : — 

*  "  Die  motlcrnen  Tbeorien  der  Chemie,"  Lothar  Meyer. 


GASEOUS    ELEMENTS    AND    COMPOUNDS. 


45 


d. 

</X28.88 

jr. 

Constituects. 

Hydrochloric  acid   . 

1.247 

36 

36.5 

1  part  hydrogen, 

35.5  parts  chlorine. 

Hydrobromic  acid 

80.  75 

1  part  hydrogen 

80  parts  bromine. 

Hydriodic  acid    .     . 

4.443 

128 

128 

1  part  hydrogen, 

127  parts  iodine. 

Water                   .     . 

0.623 

17.99 

18 

16            oxy°"en 

2            hydrogen. 

Hydrogen  sulphide 

1.191 

34.4 

34 

32           sulphur, 

2           hydrogen. 

Sulphurous  oxide    . 

2.247 

64.9 

64 

32            sulphur, 

32            oxygen. 

Sulphuric  oxide  .     . 

3.01 

86.9 

80 

32           sufphur, 

48            oxygen. 

Sulphuryl  chloride  . 

4.67 

134.8 

135 

32            s,ulphur, 

32            oxygen, 

71            chlorine. 

H3rdroorGn  selenide 

80 

78           selenium 

2            hydrogen. 

Selenous  oxide     .     . 

4.03 

116 

110 

78           selenium, 

32            oxygen. 

Hydrogen  telluride  - 

130 

128           tellurium, 

2            hydrogen. 

Ammonia  

0.597 

17.2 

17 

14            nitrogen, 

3            hydrogen. 

Nitric  oxide    .     . 

1.039 

30.0 

30 

14           nitrogen, 

16            oxygen. 

Nitrous  oxide      .     . 

1.520 

43.9 

44 

28            nitrogen, 

16            oxygen. 

Phosphine       .     .     . 

1.18 

34.1 

34 

31            phosphorus, 

3            hydrogen. 

Phosphorous  chloride 

4.88 

140.9 

137.5 

31            phosphorus, 

106.5        chlorine. 

Phosphorus    oxichlo- 

5.40 

155.9 

153.5 

31            phosphorus, 

ride 

16           oxygen,  s 

106.5        chlorine. 

Phosphorus     sulpho- 

5.88 

169.7 

169.5 

31            phosphorus, 

chloride 

32           sulphur, 

106.5        chlorine. 

Triethylphosphine 

4.60 

132.8 

134 

31           phosphorus, 

oxide 

16            oxygen, 

72            carbon, 

15            hydrogen. 

Arsine   . 

2.695 

77.8 

78 

75            arsenic, 

3            hydrogen. 

Arsenous  chloride    . 

6.30 

181.9 

181.5 

75            arsenic, 

106.5  "    chlorine. 

DISCUSSION    OF    ATOMS    AND    MOLECULES. 


cf. 

dX28.88 

M. 

Constituents. 

Cacoctyl  chloride 

4.56 

131.7 

140.5 

75  parts  arsenic, 

35.5 

'    chlorine, 

. 

24 

"    carbon, 

6 

"    hydrogen. 

Cacodyl  cyanide 

4.63 

133.7 

131 

75 

"    arsenic, 

14 

"    nitrogen, 

36 

"    carbon, 

6 

"    hydrogen. 

Arsenous  iodide  .     . 

16.1 

464.8 

456 

75 

44    arsenic, 

381 

44    iodine. 

Antimonous  chloride 

7.8 

224.7 

228.5 

122 

"    antimony, 

106.5 

"    chlorine. 

Triethylstibine    . 

7.44 

214.8 

209 

122 

44    antimony, 

72 

"    carbon, 

15 

"    hydrogen. 

Bismuthous  chloride 

11.35 

327.7 

316.5 

210 

44    bismuth, 

106.5 

"    chlorine. 

Boric  chloride     .     . 

3.942 

113.8 

117.5 

11 

"    boron, 

106.5 

"    chlorine. 

Boric  fluoride      .     . 

2.312 

66.8 

68 

11 

44    boron, 

57 

"    fluorine. 

Boric  bromide     .     . 

8.78 

253.5 

251 

11 

"    boron, 

240 

"    bromine. 

Trimetliylborine 

1.93 

55.7 

56 

11 

44    boron, 

36 

44    carbon, 

9 

"    hydrogen. 

Marsh-gas  .... 

0.557 

16.1 

16 

12 

"    carbon, 

4 

"*  hydrogen. 

Methyl  fluoride   .     . 

1.186 

34.3 

34 

12 

"    carbon, 

19 

"    fluorine, 

3 

u    hydrogen. 

Methyl  chloride  .     . 

1.736 

50.1 

50.5 

12 

44    carbon, 

35.5 

"    chlorine, 

3 

44    hydrogen. 

Methyl  bromide  .     . 

3.253 

93.9 

95 

12 

"    carbon, 

80 

44    bromine, 

3 

"    hydrogen. 

Methyl  iodide      .     . 

4883 

141 

142 

12 

"    carbon, 

127 

"    iodine, 

3 

44    hydrogen. 

Chloroform     .     .     . 

4.2 

121.3 

119.5 

12 

44    carbon, 

1 

44    hydrogen, 

106.5 

44    chlorine. 

Carbon  tetrachloride 

5.24 

151.3 

154 

12 

44    carbon, 

142 

4'    chlorine. 

Carbon  monoxide    . 

0.968 

27.96 

28 

12 

"    carbon, 

16 

44    oxygen. 

Carbon  dioxide   . 

1.529 

44.15 

44 

12 

"    carbon, 

32 

"    oxygen. 

GASEOUS  ELEMENTS  AND  COMPOUNDS. 


47 


d. 

dX2S.SS 

M. 

Constituents. 

Carbon  oxichloricle  . 

3.505 

101.2 

99 

12 

parts  carbon, 

16 

oxygen, 

71 

chlorine. 

Carbon  oxisnlphicle  . 

2.105 

60.8 

60 

12 

carbon, 

16 

oxygen, 

32 

sulphur. 

Carbon  sulphide  .     . 

2.645 

76.4 

76 

12 

carbon, 

64 

sulphur. 

Hydrocyanic  acid    . 

0.948 

27.4 

27 

12 

carbon, 

14 

nitrogen, 

1 

hydrogen. 

Cyanogen  chloride  . 

2.13 

61.5 

61.5 

12 

carbon, 

14 

nitrogen, 

35 

5        chlorine. 

Cyanic  acid     .     .     . 

1.50 

43.3 

43 

12 

carbon, 

14 

nitrogen, 

16 

oxygen, 

1 

hydrogen. 

Methyl  alcohol    . 

1.12 

32.3 

32 

12 

carbon, 

16 

oxygen, 

4 

hydrogen. 

Methyl  nitrate    .     . 

2.64 

76.2 

77 

12 

carbon, 

14 

nitrogen, 

3 

hydrogen, 

48 

oxygen. 

Silicic  fluoride 

3.57 

103.0 

104 

28 

silicon, 

76 

fluorine. 

Silicic  chloride     . 

5.94 

171.5 

170 

28 

silicon, 

142 

chlorine. 

Silicic  iodide  .     .     . 

19.1 

551.4 

536    !  28 

silicon, 

508 

iodine. 

Siliconethyl    .     .     . 

5.13 

148.1 

144 

28 

silicon, 

96 

carbon, 

20 

hydrogen. 

Titanic  chloride  .     . 

6.84 

197.5 

192 

50 

titanium, 

142 

chlorine. 

Zirconic  chloride 

8  15 

235.4 

232 

90 

zirconium, 

U2 

chlorine. 

Stannic  chloride 

9.20 

265.7 

260    118 

tin, 

1^2 

chlorine. 

Tinethyl     .... 

8.02 

231.6 

234    118 

tin, 

96 

carbon, 

20 

hydrogen. 

Tintriethyl  chloride 

8.43 

243.4 

240.5  118 

tin, 

35. 

5        chlorine, 

72 

carbon, 

15 

'    hydrogen. 

48 


DISCUSSION    OF    ATOMS    AND    MOLECULES. 


d. 

^X28-88       M-                   Constituents. 

Tintriethyl  bromide 

9.92 

286.4      285 

118  parts 

tin, 

80 

bromine, 

72        ' 

carbon, 

15        ' 

hydrogen. 

Tintriethyl  iodide    . 

10.33 

298.2 

290 

118       ' 

tin, 

127 

iodine, 

36       ; 

carbon, 

9       ' 

hydrogen. 

Leadmethyl    .     .     . 

9.6 

277.2 

267 

207       ' 

lead, 

48       ' 

carbon, 

12       ' 

hydrogen. 

Zincmethyl     .     .     . 

3.29 

95.0 

95.2 

65.2    ' 

zinc, 

24       ' 

carbon, 

6        ' 

hydrogen. 

Ziucethyl   .... 

4.62 

123 

123.2 

65.2    ' 

zinc, 

48        ' 

carbon, 

10 

hydrogen. 

Mercurymethyl  .     . 

8.29 

239.4 

230 

200 

mercury, 

24       * 

carbon, 

6       ' 

hydrogen. 

Mercury  ethyl      .     . 

9.97 

287.8 

258 

200 

mercury, 

48        ; 

carbon, 

10 

hydrogen. 

Mercuric  chloride    . 

9.8 

283 

271 

200 

mercury, 

71 

chlorine. 

Mercuric  bromide     .   j  12.16 

3.51 

360 

200 

mercury, 

160      " 

bromine. 

Mercuric  iodide  .     . 

16.2 

468 

454 

200      " 

mercury, 

254      k' 

iodine. 

Osmic  oxide    .     .     . 

8.9 

257 

263.2 

199.2  " 

osmium, 

64      " 

oxygen. 

Chromic  acichloride 

5.55 

159 

155 

52      " 

chromium, 

32      " 

oxygen, 

71       " 

chlorine. 

Molybdenic  chloride 

9.46 

273 

269.5 

92      " 

molybde- 

num, 

177.5  " 

chlorine. 

Tungsten  chloride  . 

12.7 

366 

361.5 

184      " 

tungsten, 

177.5  " 

chlorine. 

Tungsten     hexachlo- 

13.2 

382 

397 

184      " 

tungsten, 

ride 

213      " 

chlorine. 

Tungsten  oxichloride  11.84 

342 

342 

184      " 

tungsten, 

16      " 

oxygen, 

142       4 

chlorine. 

Vanadic  tetrachloride 

6.69 

193 

196.3 

51.3    ' 

vanadium, 

142       ' 

chlorine. 

Vanadic  acichloride 

6.11 

176 

173.8 

51.3    ' 

vanadium, 

16       ' 

oxygen, 

106.5    ' 

chlorine. 

GASEOUS    ELEMENTS    AND    COMPOUNDS. 


49 


d. 

rfX2S.SS 

M. 

Constituents. 

Niobic  chloride  .  . 
Niobic  acichloride  . 

Tantalic  chloride  . 

9.6 

7.88 

12.9 

277 
228 

372 

272.5 
217.5 

359.5 

05  parts  niobium, 
177.5  "    chlorine. 
95      "    niobium, 
16      "    oxygen, 
106.5  "    chlorine. 
182      "    tantalum, 
177.5  "    chlorine. 

Aluminic  chloride  . 
Aluminic  bromide  . 
Aln  min  5c  iodide  .  . 
Ferric  chloride  .  . 

9.35 
18.G 
27 
11.39 

270 

537 
780 
329 

267.6 
534.6 
816.6 
325 

54.6  "    aluminium, 
213      "    chlorine. 
54.6  "    aluminium, 
480      "    bromine. 
54.6  "    aluminium, 
762      "    iodine. 
112      "    iron, 
213      "    chlorine. 

This  list  contains  compounds  of  thirty  different  ele- 
ments; and,  by  means  of  these  compounds,  supposing 
that  they  contain  at  least  one  of  their  elements  in  the 
smallest  possible  quantity,  we  can  determine  the  atomic 
weights  of  the  elements  concerned.  It  will  be  seen,  how- 
ever, that,  whereas  the  hypothesis  of  Avogadro  furnishes 
us  with  a  principle  which  enables  us  to  state  positively 
what  the  molecular  weight  of  any  gaseous  compound  is, 
it  does  not  furnish  a  means  for  the  determination  of 
atomic  weights  directly.  After  examining  the  various 
compounds  of  an  element,  we  select  that  one  which  con- 
tains the  smallest  quantity  of  the  element  in  the  mole- 
cule, and  then,  without  proof  of  any  kind,  we  say  this 
smallest  quantity  represents  the  atom.  Thus  it  is  evident 
that  the  atomic  weights,  as  determined  by  this  method, 
rest  upon  a  more  or  less  doubtful  basis.  For  practical 
purposes,  however,  this  is  not  a  serious  matter;  inasmuch 
as,  although  we  cannot  assert  that  the  weight  found 
really  represents  the  atomic  weight,  we  can  assert  that 
it  represents  the  weight  of  that  portion  of  the  element 
which  conducts  itself  as  an  atom,  i.e.,  throughout  the 
whole  series  of  changes  which  it  undergoes  in  its  com- 
pounds it  is  indivisible. 

Other  Proofs  of  the  Fact  that  the  Molecule*  of  Ele- 
ments contain  more  than  one  Atom. — It  has  been  stated 
5 


50          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

above  that  the  molecules  of  elements  contain,  in  almost 
all  cases,  more  than  one  atom  ;  and  it  has  been  shown 
that,  if  the  hypothesis  of  Avogadro  be  accepted,  we  are 
necessarily  led  to  this  conclusion  by  a  simple  considera- 
tion of  the  molecular  weights  of  elements  and  their  com- 
pounds. The  question  naturally  arises:  Are  there  other 
evidences,  independent  of  Avogadro's  hypothesis,  of  the 
fact  that  the  molecules  of  elements  consist  of  more  than 
one  atom  ?  There  are. 

A  number  of  the  elements,  as  we  ordinarily  meet  with 
them  in  the  free  state,  conduct  themselves  as  compara- 
tively neutral  bodies.  Take,  for  instance,  hydrogen.  As 
a  gas,  this  element  possesses  very  little  affinity  for  most 
other  elements  and  compounds.  We  may  bring  it  in 
immediate  contact  with  most  substances  without  effecting 
any  change  whatever  in  these  substances.  If,  however, 
we  set  it  free  from  one  of  its  compounds,  and,  at  the 
moment  it  is  set  free,  allow  it  to  act  upon  some  other 
body,  we  find  that  it  is  a  comparatively  active  element, 
capable  of  effecting  very  material  changes  in  other  sub- 
stances. The  same  is  true  of  oxygen,  nitrogen,  and  other 
elements.  They  are  much  more  active  in  the  status 
nascendi  than  in  the  free  state.  How  shall  we  explain 
this  ?  This  can  be  done  most  readily  by  supposing  the 
molecules  of  these  elements,  in  the  free  state,  to  contain 
more  than  one  atom  bound  together  by  chemism.  Now, 
if  it  be  required  that  an  element  thus  constituted  shall 
combine  with  another  body,  it  is  first  necessary  that  the 
force  which  holds  together  the  atoms  be  overcome ;  the 
atoms  must  be  torn  asunder  before  they  can  act  as 
atoms ;  or,  in  other  words,  a  decomposition  must  be 
effected  before  the  required  combination  can  take  place. 

Sometimes  the  attraction  exerted  by  an  atom  of  one 
element  for  an  atom  of  another  is  so  strong  that  this 
decomposition  is  effected,  arid  the  combination  then  takes 
place.  Thus,  if  we  bring  hydrogen  and  chlorine  together, 
both  in  the  free  state,  they  combine.  In  this  case,  the 
chlorine  atom  attracts  the  hydrogen  atom  more  strongly 
than  the  hydrogen  atom  attracts  its  fellow,  or  the  chlo- 
rine atom  its  own  chlorine  atom.  On  the  other  hand, 
numerous  instances  of  the  opposite  kind  might  be  ad- 
duced. One  will  suffice  for  the  purpose  of  illustration. 
When  hydrogen  gas  is  passed  through  nitric  acid  no 


GASEOUS    ELEMENTS    AND    COMPOUNDS.  51 

change  takes  place.  But,  if  we  dissolve  zinc  in  nitric 
acid,  a  portion  of  the  acid  is  decomposed  by  the  hydrogen 
evolved.  The  hydrogen  atoms,  set  free  from  the  nitric 
acid,  find  the  acid  present,  and  act  upon  it  in  preference 
to  combining  to  form  free  hydrogen ;  the  elements  in 
combination  with  nitrogen  are  forcibly  removed  and  the 
hydrogen  takes  their  place,  forming  ammonia. 

Ordinary  oxygen  contains  two  atoms  in  the  molecule. 
Ozone,  another  variety  of  oxygen,  has  the  density  1.658, 
from  which  we  find  its  molecular  weight  =  48.  Now  as 
the  atomic  weight  of  oxygen,  according  to  previous  de- 
terminations, is  16,  it  follows  that  the  molecule  of  ozone 
contains  three  atoms.  The  difference  between  the  two 
forms  of  oxygen  is  thus  readily  seen.  Ozone  is  com- 
paratively unstable.  It  gives  up  its  extra  atom,  with 
great  ease  to  bodies  with  which  it  comes  in  contact,  and 
causes  thus  an  energetic  oxidation.  When  heated  up  to 
300°,  it  is  also  decomposed,  forming  ordinary  oxygen, 
and  then  an  increase  of  volume  is  observed.  In  this 
case,  if  no  foreign  body  be  present  with  which  the  libe- 
rated atom  can  unite,  it  unites  with  another  atom  of  the 
same  kind.  When  it  acts  upon  other  bodies,  the  original 
volume  of  the  gas  remains  unchanged.  It  would  then 
appear  that,  in  the  molecule  of  ozone,  two  of  the  atoms 
are  held  together  by  a  stronger  force  than  that  which 
binds  the  third  characterizing  atom  ;  or,  rather,  that  the 
two  atoms  of  the  molecule  of  ordinary  oxygen  are  held 
together  more  firmly  than  the  three  atoms  in  the  mole- 
cule of  ozone.  Here  then  again  the  different  actions  of 
the  two  varieties  can  be  best  explained  by  supposing  in 
each  case  the  molecule  to  consist  of  more  than  one  atom. 

These  and  other  similar  considerations  serve  to 
strengthen  the  conclusion  which  was  logically  deduced 
from  Avogadro's  hypothesis,  and  hence,  in  turn,  to  in- 
crease the  probability  that  the  hypothesis  is  tiie  expres- 
sion of  a  natural  law. 

Molecular  Formulas  of  Gaseous  Compounds. — When 
the  atomic  weights  of  the  elements  are  once  determined, 
the  rule  of  Avogadro,  taken  in  conjunction  with  analysis, 
is  sufficient  to  enable  us  to  establish  the  molecular  for- 
mulas of  gaseous  chemical  compounds;  a  problem,  the 
solution  of  which  without  this  rule  is  in  many  cases 


52          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

exceedingly  difficult,  and,  indeed,  at  times  impossible. 
Let  us  suppose  the  atomic  weights  of  carbon  (12),  hydro- 
gen (1),  and  oxygen  (16)  to  be  known.  We  analyze  a 
certain  compound,  and  find  that  it  contains  37.50  per  cent, 
carbon,  12.50  per  cent,  hydrogen,  and  50.00  per  cent, 
oxygen.  This  gives  the  atomic  proportion  1  of  carbon, 
4  of  hydrogen,  and  1  of  oxygen  ;  and  hence  the  simplest 
formula  we  can  assign  to*  the  compound  is  CH+0.  But 
there  is  nothing  thus  far  to  prevent  our  acceptance  of 
one  of  the  formulas  C2H80,  or  C.^H^O^,  etc.,  all  of  which 
satisfy  the  results  of  the  analysis.  We  now  determine 
the  molecular  weight  by  Avogadro's  rule,  and  find  it  to 
be  32 ;  and,  as  the  sum  of  the  weights  of  the  atoms  in  a 
molecule  of  a  compound  of  the  formula  CH40  is  32,  we 
recognize  this  latter  as  the  correct  formula. 

Apparent  Exceptions. — All  formulas  of  chemical  com- 
pounds at  present  emplo^yed  are  intended  to  represent 
molecules  of  the  compounds.  They  are  molecular  for- 
mulas. They  represent  those  amounts  of  the  bodies 
which,  in  a  gaseous  condition,  would  occup}^  the  same 
space  as  a  molecule  of  hydrogen.  If  we  take  two 
volumes  of  hydrogen  as  the  standard  of  comparison, 
then  the  formulas  of  compounds  represent  two  volumes 
of  the  same  size.  To  this  there  are  apparent  exceptions. 
When  ammonia  acts  upon  hydrochloric  acid,  the  two 
gases  combine  in  the  proportion  of  1  vol.  of  the  one  to 
1  vol.  of  the  other,  forming  a  solid  compound  which  con- 
tains 26.17  per  cent,  nitrogen,  7.48  per  cent,  hydrogen, 
and  66.35  per  cent,  chlorine.  This  gives  the  atomic  pro- 
portion 1  nitrogen,  4  hydrogen,  and  I  chlorine ;  and  the 
simplest  formula  we  can  assign  to  the  compound,  provided 
the  atomic  weights  of  nitrogen,  hydrogen,  and  chlorine  are 
respectively  14,  l,and  35. 5,  is  NH4C1.  On  now  determin- 
ing the  molecular  weight  by  Avogadro's  rule,  this  is  found 
to  be  26.75,  or  half  of  that  required  by  the  above  formula. 
Evidently,  it  is  impossible  for  a  molecule  made  up  of 
chlorine,  nitrogen,  and  hydrogen,  with  the  atomic  weights 
above  assigned  to  them,  to  have  as  small  a  weight  as 
26.75 ;  and,  to  satisfy  the  results  of  this  determination, 
we  would  be  obliged  to  write  the  formula  Ni  H2C1^  ,  and 
thus  accept  the  existence  of  half-atoms,  which,  according 
to  the  definition  of  atoms,  is  impossible.  We  might  also 


GASEOUS    ELEMENTS    AND    COMPOUNDS.  53 

imagine  the  atomic  weight  of  nitrogen  and  chlorine,  as 
already  determined,  to  be  just  twice  too  great;  for,  if  we 
assign  to  nitrogen  the  atomic  weight  7,  and  to  chlorine 
17.7,  we  could  write  the  formula  of  the  compound  NH,C1, 
and  this  compound  would  have  the  molecular  weight 
26.7,  as  determined  by  Avogadro's  method.  On  the 
other  hand,  if  7  is  the  atomic  weight  of  nitrogen,  and 
17.7  that  of  chlorine,  then  in  all  other  compounds  of 
nitrogen  or  chlorine,  in  which  one  atom  has  been  sup- 
posed to  exist  in  the  molecule,  we  must  necessarily 
accept  the  existence  of  two  atoms  in  the  molecule.  But 
then  all  these  compounds  would  not  come  under  Avoga- 
dro's rule.  Hence  we  see  that  the  compound  NH4C1 
appears  to  be  an  exception,  and,  if  no  satisfactory  expla- 
nation can  be  found  to  account  for  this  case,  its  existence 
is  fatal  to  the  rule.  A  satisfactory  explanation  can  be 
offered,  however,  as  follows. 

If  it  can  be  proved  that  the  vapor  obtained  from  the 
compound  NH^Cl  is  not  the  vapor  of  this  compound,  but 
a  mixture  of  the  vapors  of  its  constituents  NH3  and 
HC1,  the  case  becomes  a  very  simple  one.  Without 
entering  into  details,  it  may  be  mentioned  that  the  results 
of  the  experiments  made  upon  this  subject  have  justified 
the  assumption  that,  when  the  compound  NH4C1  is  heated 
up  to  a  temperature  sufficiently  high  to  cause  its  conver- 
sion into  vapor,  it  becomes  resolved  into  its  constituents 
NH,  and  HC1;  and  that,  when  this  mixture  of  two  vapors 
is  again  cooled  down,  the  two  again  unite  to  form  the 
original  compound. 

As  NH.,  and  HCl  combine  volume  to  volume,  so  the 
mixture  of  the  two  gases,  obtained  by  heating  NH+C1, 
consists  of  equal  volumes  of  the  two ;  and  the  specific 
gravity  of  this  mixed  vapor  would  be  the  mean  of  the 
specific  gravities  of  its  constituents.  The  specific  gravity 
of  NH3  is  0.597,  that  of  HCl  is  1.247  ;  the  specific  gravity 

of  a  mixture  of  the  two  would  be  °'597  +  L24T  =  0.922; 

2i 

and  this  specific  gravity,  if  supposed  to  be  that  of  a  che- 
mical compound,  would  lead  to  the  molecular  weight  26. G. 
The  number  0.922  is,  indeed,  that  found  as  the  specific 
gravity  of  the  vapor  of  NH4C1,  and  it  will  thus  be  seen 
that  the  fact  can  be  satisfactorily  explained  without 

5* 


54          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

giving  up  our  belief  in  the  correctness  of  Avogadro's 
hypothesis. 

The  compound  PCL  was  also,  at  one  time,  supposed  to 
form  an  exception  to  the  rule.  The  specific  gravity  of 
its  vapor  was  found  to  be  3.66,  from  which  was  calculated 
the  molecular  weight  105.7,  half  that  required  by  the 
formula  PCL.  It  has  been  shown,  however,  that  above  a 
certain  temperature  the  molecule  of  PCL  breaks  up  into 
a  molecule  of  PC1.{  and  a  molecule  of  C12.  The  vapor 
from  PCI.  is  a  mixture  of  two  vapors,  and  the  mean  spe- 
cific gravity  of  the  two  is  the  specific  gravity  found.  The 
specific  gravity  of  the  vapor  of  PC13  is  4.88 ;  that  of  Cl 

is  2.45;  mean  specific  gravity  —  f2-45^  3.666.        In 

this  case  it  has  further  been  shown  that  the  breaking  up 
of  the  molecule  takes  place  gradually;  for  the  specific 
gravity  of  the  vapor  decreases  from  5.08  to  3.66,  as  the 
temperature  is  elevated  from  182°  to  300°,  at  which  latter 
temperature  the  decomposition  appears  to  be  complete, 
no  further  decrease  in  specific  gravit}7  being  noticed  when 
the  vapor  is  heated  still  higher. 

It  has  further,  recentty,  been  shown  that  when  the  spe- 
cific gravity  of  the  vapor  of  PC15  is  determined  in  the 
presence  of  PC13,  its  dissociation  is  prevented,  and  the 
specific  gravity  found  is  that  which  theory  requires  for  a 
compound  of  the  formula  PC15.  The  mean  result  of  seven 
determinations  was  1-226,  whereas,  the  theory  requires 
7.217.  From  this  it  is  evident  that  the  hypothesis  of 
Avogadro  is  as  valid  for  the  compound  PCL  as  for  other 
compounds. 

A  third  example  of  this  kind  of  decomposition  by  in- 
crease of  temperature  is  met  with  in  the  case  of  the  com- 
pound N0.z.  This  body,  at  a  low  temperature,  consists 
of  colorless  ciTstals.  At  a  slightly  elevated  temperature 
these  crystals  change  to  a  yellow  liquid.  The  liquid  boils 
at  20-30°,  and  is  then  converted  into  a  gas  of  a  reddish- 
brown  color,  and,  as  the  temperature  of  the  gas  increases, 
the  intensity  of  the  color  also  increases.  The  specific 
gravity  of  the  gas  decreases  with  this  elevation  of  tem- 
perature ;  hence,  it  is  supposed  that  the  body,  at  a  low 
temperature,  must  be  represented  by  the  formula  N2O4, 
but  that  the  molecule  is  broken  up  by  an  increase  of  tem- 
perature, forming  two  molecules  of  K02.  The  latter  is 


GASEOUS    ELEMENTS    AND    COMPOUNDS.  55 

strongly  colored,  and  the  more  of  it  there  is  present  in  the 
mixture,  the  more  intense  will  be  the  color  of  the  gas. 

Among  chemical  compounds  there  are  very  few  indeed 
that  conduct  themselves  in  the  manner  of  the  three  just 
described.  As  regards  some  of  these,  good  proof  can 
be  given  of  the  fact  that  their  molecules  are  broken  up 
by  the  conversion  into  vapor,  and  hence,  the  apparently 
abnormal  specific  gravities  observed  for  these  vapors  find 
a  simple  explanation.  As  regards  others,  although  posi- 
tive proof  to  the  same  effect  may  indeed  be  lacking  as 
yet,  still  strong  indications  are  presented  that  the  abnor- 
mal densities  may  be  referred  to  the  same  cause.  So  that, 
up  to  the  present,  not  only  is  no  fact  known  that  speaks 
strongly  against  Avogadro's  hypothesis,  but,  on  the  con- 
trary, new  developments,  increased  knowledge  are  con- 
stantly tending  to  strengthen  it.  It  forms,  to-day,  by  far 
the  most  reliable  means  for  the  determination  of  molecular 
weights  of  compounds  and  elements ;  and  we  have  seen 
how,  secondarity,  it  aids  us  in  determining  atomic  weights. 
But,  in  order  that  it  may  be  useful,  it  is  necessary  that 
the  compound  which  we  desire  to  study  shall  be  capable 
of  conversion  into  vapor,  or,  if  it  be  an  element  under 
consideration,  that  at  least  one  of  the  compounds  of  this 
element  be  gaseous  or  volatile.  Only  a  comparatively 
small  number  of  compounds  satisfy  these  conditions,  and 
of  the  64  elements,  only  30  (see  List,  pp.  45-49)  enter 
into  the  composition  of  these  compounds.  With  no  other 
means,  then,  at  our  command,  the  work  would  be  incom- 
plete. It  is  necessary  that  some  other  method  should  be 
introduced  which  shall  be  applicable  to  those  elements  not 
covered  by  Avogadro's  rule,  i.  e.,  those  elements  which 
are  themselves  incapable  of  conversion  into  vapor,  and 
which  do  not  enter  into  the  composition  of  gaseous  or 
volatile  compounds. 


III. 

EXAMINATION  OF  SOLID  ELEMENTS  AND 
COMPOUNDS. 

Specific  Heat. — It  has  been  noticed  that,  when  equal 
weights  of  different  substances  are  exposed  to  the  heat 
from  the  same  source,  they  will  have  different  tempera- 
tures at  the  end  of  the  same  period  of  time.  From  this 
we  conclude  that,  to  raise  equal  weights  of  different  sub- 
stances through  the  same  number  of  degrees  of  tempera- 
ture, different  quantities  of  heat  are  necessary.  Given 
exactljr  the  same  heating  power,  it  takes  about  32  times 
as  long  to  raise  the  temperature  of  a  pound  of  water  ten, 
twenty,  or  thirty  degrees,  as  it  takes  to  raise  the  tempe- 
rature of  a  pound  of  mercury  the  same  number  of  degrees  ; 
or  it  takes  32  times  as  much  heat  to  raise  the  temperature 
of  a  pound  of  water  ten,  twenty,  or  thirty  degrees  as  it 
takes  to  raise  the  temperature  of  the  pound  of  mercury 
the  same  number  of  degrees.  The  quantity  of  heat  re- 
quired to  raise  the  temperature  of  a  given  weight  of  any 
substance  a  given  number  of  degrees,  as  compared  with 
the  quantity  of  heat  required  to  raise  the  temperature 
of  the  same  weight  of  water  the  same  number  of  degrees, 
is  called  the  specific  heat  of  the  substance.  The  quantity 
of  heat  required  to  raise  the  temperature  of  a  pound  of 
water  one  degree  Centigrade,  may  be  conveniently  adopted 
as  the  thermal  unit.  We  then  speak  of  the  specific  heat 
of  water  as  =  1 ;  and  the  specific  heat  of  any  other  body 
is  the  relative  quantity  of  heat  necessary  to  raise  the 
temperature  of  a  pound  of  this  body  one  degree  Centi- 
grade, taking  the  above  thermal  unit  as  the  standard. 
The  specific  heat  of  mercury,  according  to  the  results  of 
the  experiment  mentioned,  is  0.03332 ;  that  of  gold  is 
found  to  be  0.03244,  etc.  etc.  The  meaning  of  these 
numbers  will  be  readily  seen. 


SOLID    ELEMENTS    AND    COMPOUNDS/  57 

Relations  between  Specific  Heat  and  Atomic  Weight. — 
Now,  when  the  solid  elements  are  examined  with  reference 
to  their  specific  heats,  a  very  simple  relation  is  found  to 
exist  between  the  numbers  expressing  the  specific  heats 
and  the  atomic  weights.  This  relation  will  be  made  clear 
by  a  consideration  of  a  few  cases  actually  examined  : — 

Element.  Specific  heat.        Atomic  weight, 

Silver  .    .    ...    .  0.0570  108 

Zinc    .    .    .    .    .  0.0955       65.2 

Cadmium        ..      ;        .         .  0.0507  112 

Copper 0.0952  63.4 

Tin 0.0562  118 

It  will  be  seen  by  an  examination  of  this  table  that  the 
atomic  weights  are  inversely  proportional  to  the  specific 
heats.  We  have — 

108  :  65.2  :  :  0.0955  :  0.0570; 

112  :  63.4  :  :  0.0952  :  0.0567; 

108  :  118  :  :  0.0562  :  0.0570,  etc.  etc. 

These  proportions  are  only  approximately  correct ; 
but  it  must  be  remembered  that  the  means  for  the 
determination  of  atomic  weights  are  capable  of  much 
greater  refinement  than  those  employed  for  the  determi- 
nation of  specific  heats.  There  is  much  greater  liability 
of  error  in  the  latter  determinations  than  in  the  former. 
Hence  such  slight  variations  from  absolute  agreement  in 
these  proportions  can  occasion  no  surprise.  The  agree- 
ment is  sufficiently  close  to  indicate  a  decided  and  un- 
doubted connection  between  the  two  sets  of  numbers. 
This  connection  may  be  stated  in  another  way:  The 
product  of  the  atomic  weight  into  the  specific  heat  is  a 
constant  quantity  for  the  elements  examined.  Thus  in 
the  above  cases: — 

108     X  0.057   =6.15 
65.2x0.0955  =  6.22 

112     X  0.0567  =  6.35 
63.4  x  0.0952  =  6. 03 

118     X  0.0562  =  6.63 

For  the  same  weights,  then,  the  quantities  of  heat 
necessary  to  elevate  the  temperature  of  the  elements  one 
degree  vary.  The  quantity  necessary  to  elevate  the 
temperature  of  an  atom  one  degree  would,  of  course,  be 
represented  by  the  variable  quantity  multiplied  by  the 
atomic  weight ;  and  this  product,  in  the  cases  cited,  we 
find  to  be  represented  by  a  constant. 


58  DISCUSSION    OF    ATOMS    AND    MOLECULES. 

Investigations  of  Dulong  and  Petit. — In  the  year  1819  at- 
tention was  first  called  to  the  above  relation  by  Dulong  and 
Petit ;  and,  having  examined  a  large  number  of  elements, 
they  felt  justified  in  propounding  the  law:  The  atoms  of 
all  elementary  bodies  have  exactly  the  same  capacity  for 
heat.  This  is  simply  a  generalization  from  the  facts 
stated,  and  is  another  way  of  stating  that,  to  raise  the 
temperature  of  atoms  one  degree,  the  same  quantity  of 
heat  is  always  necessary. 

If  the  law  propounded  is  in  reality  a  law,  it  will  be 
readily  seen  that  a  new  means  is  given  for  the  determina- 
tion of  the  atomic  weights  of  elements  of  which  we  can 
know  the  specific  heat.  If  we  assume  that  the  constant 
number  obtained  by  multiplying  the  specific  heats  by  the 
atomic  weights  is  6.25,  which  is  about  the  average  of  the 
different  values  found,  then  it  is  plain  that,  if  we  divide 
this  number  by  the  specific  heat  of  an  element,  we  shall 
obtain  a  number  which  approximately  represents  the 
atomic  weight.  If  we  call  the  atomic  weight  A,  and  the 
specific  heat  H,  the  following  formula  will  express  the 
relation : — 

A  —  — 25 

In  order  that  this  might  hold  good  for  all  the  elements 
investigated  bjT  Dulong  and  Petit,  they  found  it  necessary 
to  change  the  atomic  weights  of  four  of  the  metals ;  just 
as  it  had  been  necessaiy  to  change  certain  of  the  atomic 
weights  in  order  that  Avogadro's  hypothesis  might  hold 
good  in  all  cases.  But,  as  these  atomic  weights  had 
been  determined  purely  empirically,  and  thus  rested 
upon  a  questionable  basis,  there  could  be  no  serious 
objection  to  the  change.  Notwithstanding  the  simplicity 
of  the  law,  its  validity  was  not,  however,  immediately 
acknowledged. 

Investigations  of  Neumann  and  Regnault. — Twelve 
yeas  later  (1831),  Neumann  published  investigations  on 
the  specific  heat  of  chemical  compounds,  and  showed 
that,  for  bodies  of  similar  composition,  the  specific  heats 
are  inversely  proportional  to  the  molecular  weights  of 
the  compounds;  or  the  molecules  of  different  compounds 
have  equal  capacity  for  heat ;  i.e.,  for  bodies  of  similar 


SOLID    ELEMENTS    AND    COMPOUNDS.  59 

composition,  the  product  of  the  molecular  weight  (M) 
into  the  specific  heat  (H]  is  a  constant  quantity.  For 
example,  the  specific  heat  of  lead  iodide  is  0.0427 ;  that 
of  lead  bromide  is  0.0533;  that  of  lead  chloride  is  0.0664; 
the  molecular  weights  of  these  compounds  are  respec- 
tively 461,  367,  and  278.  The  products  MXH  are  as 
follows : — 

For  lead  iodide      .         .     461x0.0427  =  19.68 
bromide.        .     867x0.0588  =  19.56 

"        chloride.         .     278x00664  =  18.46 

Further,  the  specific  heat  of  barium  chloride  is  0.0902; 
that  of  strontium  chloride  is  0.1199;  that  of  calcium 
chloride  is  0.1642.  The  molecular  weights  of  these 
compounds  are  respectively  208,  158.5,  and  111.  The 
products  MX  //are — 

For  barium  chloride  .  208  X  0.0902  =  18.76 
"  strontium  chloride  .  158.5x0.1199  =  19.00 
"  calcium  chloride  .  Ill  x  0.1642  =  18.22 

Subsequently,  similar  investigations  were  carried  out 
in  connection  with  a  larger  number  of  compounds,  and  it 
is  particularly  to  the  labors  of  Regnault  (1840)  that  the 
development  of  this  branch  of  the  subject  is  due.  The 
result  attained  may  be  stated  concisely  thus:  The  ele- 
ments possess  essentially  the  same  specific  heat  whether 
they  exist  in  a  free  state  or  are  in  combination. 

To  show  how  this  conclusion  may  be  deduced  from 
known  facts,  let  us  take  again  the  case  of  lead  iodide. 
Lead  has  the  specific  heat  0.0307,  iodine  0.0541.  Multi- 
plying by  the  atomic  weights,  we  have  0.0307  X  207  = 
6.35;  and  0.0541  X  127  =  6.87;  but,  as  can  be  deter- 
mined, there  are  two  atoms  of  iodine  in  the  molecule  of 
lead  iodide,  hence  the  atomic  heat  6.87  must  be  multiplied 
by  2,  which  gives  13.74.  To  raise  the  constituents  of 
lead  iodide  one  degree  in  temperature  would  then  require 
an  amount  of  heat  represented  by  the  number  6.35  -f- 
13.74  =  20.09,  and  we  have  found  that  the  amount  of 
heat  necessary  to  raise  lead  iodide  as  a  compound  one 
degree  in  temperature  is  19.68.  As  these  results  may  be 
looked  upon  as  coincident,  it  follows  that  the  specific 
heat  of  the  elements  in  this  case  is  the  same  whether  the 
elements  be  in  combination  or  in  the  free  state. 


60          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

Determination  of  Atomic  Weights  by  a  Study  of  the 
Specific  Heat  of  Compounds.  —  Adopting  the  principle 
involved  in  the  above  remarks,  we  see  that  a  study  of  the 
specific  heat  of  compound  bodies  ma}^  aid  us  in.  the 
determination  of  atomic  weights;  for  we  may  ascertain 
the  specific  heat  of  an  element  even  where  this  cannot  be 
determined  directly.  It  is  difficult,  for  instance,  to 
ascertain  the  specific  heat  of  gaseous  elements  directly, 
and  yet,  as  these  elements  form  solid  compounds,  the 
specific  heat  of  these  latter  may  be  determined,  and  thus, 
indirectly,  that  of  the  gaseous  elements. 

To  illustrate  by  an  example,  let  us  consider  the  case  of 
chlorine.  Suppose  it  be  required  to  determine  the  atomic 
weight  of  this  element  by  means  of  specific  heat  deter- 
minations. We  cannot  determine  the  specific  heat  of  the 
element  directly.  It  forms  compounds,  however,  with 
other  elements,  the  specific  heats  and  atomic  weights  of 
which  may  be  determined.  It  combines  with  lead.  The 
specific  heat  of  lead  is  found  to  be  0.0307,  which,  accord- 
ing to  the  law  of  Dulong  and  Petit,  gives  the  atomic 
weight  207.  Now,  in  lead  chloride,  we  find  that  207 
parts  by  weight  of  lead  are  combined  with  71  parts  by 
weight  of  chlorine  ;  or,  with  one  atom  of  lead,  there  is 
combined  an  amount  of  chlorine  weighing  71  times  as 
much  as  an  atom  of  hydrogen.  But  we  do  not  know  how 
many  atoms  of  chlorine  this  weight  represents.  It  cannot 
be  less  than  1,  but  it  may  be  2,  3,  4,  or  more  atoms,  as 
far  as  we  know.  We  determine  the  specific  heat  of  lead 
chloride,  and  find  it  to  be  0.0664.  We  have  assumed 
that  the  molecular  heat  of  a  compound  (i.  e.,  the  product 
of  the  molecular  weight  into  the  specific  heat)  is  equal  to 
the  sum  of  the  atomic  heats  (i.  <?.,  the  product  of  atomic 
weight  into  the  specific  heat)  of  the  atoms  contained  in 
the  compound  ;  or  — 


But  as  the  products  A  X  H,  A'  X  Hf,  A"  X  H"  have 
been  shown  to  be  constant  and  equal  to  about  6.25,  we 
may  simplify  this  equation  as  follows:  — 


and  from  this  equation,  M  and  H  being  known,  we  can 
determine  the  value  of  ??,  or  the  number  of  atoms  con- 
tained in  the  molecule. 


SOLID    ELEMENTS    AND    COMPOUNDS.  61 

In  the  case  under  consideration  we  would  have : — 

278  X  0.0664  =  w6.25; 
18.46  =n6.25; 
n  =  3. 

We  then  conclude  that  in  the  molecule  (278  parts) 
of  lead  chloride  there  are  contained  three  atoms.  But 
we  know  that  there  is  one  atom  of  lead ;  hence,  there 
must  be  two  atoms  of  chlorine  ;  and,  as  two  atoms  weigh 
71,  the  atomic  weight  of  chlorine  is  35  5,  the  same  as 
that  found  by  means  of  Avogadro's  method. 

Further,  we  have  other  compounds  in  each  of  which 
71  parts  of  chlorine  are  combined  with  a  certain  quantity 
of  another  element,  and  the  molecular  heat  of  which  is 
the  same  as  that  of  lead  chloride.  From  the  latter  fact 
we  conclude  that  there  are  also  three  atoms  contained  in 
the  molecules  of  these  compounds,  and  hence,  that  quan- 
tity of  an  element  which,  in  these  compounds,  is  com- 
bined with  71  parts  of  chlorine,  represents  the  atomic 
weight.  Thus,  we  have  for  the  molecular  heat  of  barium, 
strontium,  and  calcium  chlorides  18.76,  19.00  and  18.22, 
respectively,  numbers  which  may  be  considered  the  same 
as  18.46,  the  molecular  heat  of  lead  chloride ;  and  in 
these  compounds  there  are  137  parts  barium,  87.5  parts 
strontium,  and  40  parts  calcium,  combined  with  71  parts 
chlorine.  We  hence  conclude  that  137,  87.5  and  40  are 
respectively  the  atomic  weights  of  barium,  strontium,  and 
calcium,  although  direct  determinations  of  the  specific 
heat  have  been  made  in  only  two  of  these  cases. 

Of  course,  just  as  we  have  thus  indirectly  determined 
the  atomic  weights  of  certain  elements,  we  can  also  de- 
termine the  unknown  specific  heats  of  these  same  ele- 
ments by  a  slight  variation  of  the  process.  Take  lead 
chloride.  The  molecular  heat  of  this  compound  is  18.46  ; 
the  atomic  heat  of  lead  is  6.35  ;  hence,  we  have  18.46  — 
6.35  =  12.11,  for  twice  the  atomic  heat  of  chlorine, 
there  being  two  atoms  of  chlorine  contained  in  the 

12  11 

compound.     This  gives  the  atomic   heat  -     -  —  6.06  for 

2 

chlorine  ;  and  dividing  by  the  atomic  weight,  we  obtain 

fi  or 

— —  =0.1707,  which,  accordingly,  would   represent  the 

oO.O 


62 


DISCUSSION    OF    ATOMS    AND     MOLECULES. 


specific  heat  of  chlorine,  if  the  gases  were  subject  to  pre- 
cisely the  same  law  as  solids. 

By  processes  like  those  described,  the  atomic  weights 
of  a  number  of  the  elements  have  been  determined,  and, 
in  many  cases,  the  results  obtained  have  been  considered 
decisive. 

The  following  tables  (I.)  of  elements,  and  (II.)  of  com- 
pounds, contain  the  numbers  actually  obtained  and  the 
results  deduced  from  them.  The  numbers  under  H  are 
tliose  representing  the  specific  heats  of  the  elements ; 
those  under  A  are  the  atomic  weights  as  determined  by 
analytical  methods,  aided  by  the  rule  of  Avogadro,  or 
that  of  Dulong  and  Petit ;  finally,  in  the  last  column,  is 
the  product  of  the  atomic  weight  into  the  corresponding 
specific  heat  A  X  H,  called,  for  convenience  sake,  the 
atomic  heat. 


H. 

A. 

AXH. 

Lithium  . 

0.941 

7 

6.6 

0.293 

23 

6.7 

Magnesium     .        .        ... 

0.250 

24 

6.0 

Aluminium      .     •  . 

0.214 

27.4 

5.9 

0.173 

28 

4.8 

Phosphorus     .... 

0.174 

31 

5.4 

Sulphur  .        ... 

0.178 

32 

5.7 

Potassium    .  ,  .      .        .        . 

0.166 

39 

6.5 

Calcium           .        .        . 

0.170 

40 

6.8 

Chromium       .         .         . 

0.100 

52 

5.2 

Manganese      .         .        .        . 

0  122 

55 

6.7 

Iron         ..... 

0.114 

56 

6.4 

Cobalt     

0.107 

59 

6.2 

Nickel     

0.109 

59 

6.4 

Copper    

0.0952 

63.4 

6.0 

Zinc     *    .        .        .        . 

0.0955 

65.2 

6.2 

Arsenic                    ; 

0.0814 

75 

6.1 

Selenium         .... 

0.0746 

79.4 

5.9 

Bromine  (solid) 

0.0843 

80 

6.7 

Molybdenum  .... 

00722 

92 

6.6 

Ruthenium      .... 

0.0611 

104.4 

6.4 

Rhodium         .... 

0.0580 

104.4 

6.1 

Palladium       .... 

0.0593 

106.6 

6.3 

Silver      

0.0570 

108 

6.2 

Cadmium        .... 

0.0567 

112 

6.4 

Indium    

0.0570 

113.4 

6.5 

Tin          

0.0562 

118 

6.6 

Antimony       .... 

0.0508 

122 

6.2 

Iodine     ' 

0.0541 

127 

6.9 

SOLID    ELEMENTS    AND    COMPOUNDS 


63 


H. 

A. 

AX* 

Tellurium        .       '  . 
Tungsten 
Gold        
Platinum 

0.0474 
0.0334 
0.0324 
0.0324 
0.0326 

128 
184 
197 
197.4 
198 

6.1 
6.1 
6.4 
6.4 
6.5 

0.0311 

199.2 

6.2 

Mercury          .... 
Thallium         .... 

0.0317 
0.0335 
0.0307 

200 
204 
207 

6.3 
6.8 
6.4 

Bismuth           .... 

0.0308 

210 

6.5 

The  following  are  some  of  the  compounds  which  have 
been  employed  for  the  purpose  of  determining  the  atomic 
weights  of  elements.  The  numbers  under  H  are  those 
representing  the  specific  heats  of  the  compounds ;  those 
under  M are  the  molecular  weights  ;  the  products  MX  H 
are  the  so-called  molecular  heats ;  n  represents  the  number 
of  atoms  in  the  molecule  of  the  compound. 


H. 

M. 

MXH. 

n. 

«XH 

n 

CoAs2  .... 

Ag2S  .  .  .  .  . 

0.0920 
0.0746 

209 
248 

19.2 
18.5 

3 
3 

6.4 
6.2 

Cu,S 

0  1212 

158.8 

19.2 

8 

6.4 

Ho-S 

0  052 

232 

12.1 

9 

6.1 

NiS 

0  1281 

91 

11.6 

9 

5.8 

PbS  .  -. 

0  053 

239 

12.7 

9 

6.4 

SnS  .  . 

0  0837 

150 

12.6 

9 

6.3 

SnS2  .  .  . 

0  1193 

182 

21.7 

3 

7.2 

AgCl  .... 
CuCl  .... 
KC1  .  .  . 

0.0911 
0.1383 
0.1730 

143.5 
98.9 
74.6 

13.1 
13.7 
12.9 

2 

2 
9 

6.6 
6.9 
6.5 

LiCl  

0.2821 

42.5 

12.0 

a 

6 

NaCl  .... 
BaCl2  .... 
CaCl2  .... 
SrCl2  .  .  ^  . 
HgCl2  .... 
MgCl2  .... 
MuCl,  .... 
PbCl2  .... 

0.2140 
0.0896 
0.1642 
0.1199 
0.0689 
0.1946 
0.1425 
0.0664 

58.5 
208 
111 
158.5 
271 
95 
126 
278 

12.5 
18.6 
18.2 
19.0 
18.7 
18.5 
18.0 
18.5 

2 
3 
3 
3 
3 
3 
3 
3 

6.3 
6.2 
6.1 
6.3 
6.2 
6.2 
6.0 
6.2 

Exceptions  to  the  Law  of  Dulong  and  Petit. — On  ex- 
amining these  tables,  we  are  struck  by  the  fact  that  the 


64          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

•jur  v/    TT 

product,  Ay^Hj  in  the  first,  and  the  quotient, ,  in 

n 

the  second — although  assumed  to  be  constant  in  value — 
show  a  very  considerable  variation  from  the  mean  value, 
6.25.  For  a  large  number  of  these  cases,  we  are  justified 
in  concluding  that  the  variation  is  due  to  the  errors  of 
observation  in  consequence  of  the  imperfections  of  the 
methods  emplo}7ed  for  the  determination  of  specific  heat. 
Indeed,  in  all  the  cases  cited  in  the  above  tables,  the  vari- 
ations are  hardly  great  enough  to  lead  us  to  suspect  the 
incorrectness  of  the  law  of  Dulong  and  Petit.  If,  how- 
ever, we  consider  the  elements  carbon,  boron,  and  silicon, 
we  shall  find  that  entirely  different  results  are  reached, 
and  we  are  compelled  to  admit  that,  for  these  elements  at 
least,  the  law  does  not  appear  to  hold  good.  This  will 
be  best  seen  by  means  of  the  following  table,  in  which  H 
is  the  specific  heat ;  £,  the  temperature  at  which  the  deter- 
mination was  made ;  A,  the  atomic  weight ;  and  A  X  H, 
the  atomic  heat.  As  these  three  elements  form  the  most 
marked  exceptions  to  the  law,  all  the  most  reliable  deter- 
minations of  their  specific  heats  that  have  been  made  by 
different  observers  are  given,  for  reasons  which  we  shall 
presently  see. 


SOLID    ELEMENTS    AND    COMPOUNDS. 


65 


H. 

t. 

A. 

\Axff. 

Boron  (crystallized)     .     .     . 

it                  tt 

0.230 
0.244 

+  20  to     510 
+  10  "    100 

11 

tt 

2.5 

2.7 

Carbon  : 

a.  Diamond       .... 

0.095 

at       0 

12 

1.1 

it 

0.144 

"      50 

tt 

1.7 

u 

0.145 

+  24  to     70 

tt 

1.7 

tt 

0.147 

+    9  u      98 

tt 

1.8 

tt 

0.191 

at  100 

tt 

2.3 

tt 

0.286 

"    150 

tt 

2.8 

1  1 

0.279 

"    200 

tt 

3.3 

b.  Blast-furnace  graphite 

0.165 

+  20  to     52 

tt 

2.0 

tt                   n 

0.192 

+  23  "      66 

tt 

2.3 

1  1                    <t 

0.197 

+  12  "     98 

« 

2.4 

c.  Natural  graphite    .     . 

0.144 

0  "      34 

tt 

1.7 

tt              tt 

0.174 

+  20"      52 

it 

2.1 

tt              tt 

0.192 

+  24  "     67 

tt 

2.3 

tt              tt 

0.197 

0  "    100 

tt 

2.4 

tt              tt 

0.198 

+  15"     99 

tt 

2.4 

d    Oas-coal                 •      . 

0.185 

+  21  "     52 

« 

2.2 

tt 

0.201 

+  22  "     70 

u 

2.4 

tt 

0.200 

+  16  "   100 

1  1 

2.4 

e.  Charcoal  (veg.  )      .     . 

0.241 

+  10  "     98 

tt 

2.9 

/.                     (animal) 

0.261 

+  19"     99 

1  1 

3.1 

Silicon  : 

melted           .              •. 

0.138 

+  20  "     50 

28 

3.9 

»t 

0.166 

+  20."    100 

tt 

4.6 

crystallized 

0.165 

+  20  "     50 

tt 

4.6 

tt 

0.173 

+  20  "    100 

tt 

4.8 

*  1 

Here  then  it  will  be  observed  that  not  only  do  the 
atomic  heats  of  the  three  elements  vary  markedly  from 
the  constant  6.25,  but  for  different  conditions  of  the  same 
element  decided  variations  take  place.  In  these  cases 
there  can  be  no  thought  that  the  determinations  are  in- 
correct. They  have  been  made  repeatedly  by  the  most 
careful  e'xperimenters,  but  alwa}rs  with  the  same  results; 
and,  as  already  stated,  we  are  now  forced  to  admit  that 
the  three  elements — carbon,  boron,  and  silicon — form 
undoubted  exceptions  to  the  so-called  law  of  Dulong  and 
Petit,  as  this  law  has  been  stated.  But  shall  we  then 
without  further  inquiry  abandon  the  law  as  useless  ? 
This  question  has  been  answered  in  the  affirmative  by- 
some  ;  but  the  majority  of  chemists  at  present  accept  it 
with  the  restrictions  necessarily  imposed  by  the  nature 
of  the  facts,  and  employ  it  in  the  determination  of  atomic 

6* 


66 


DISCUSSION    OF    ATOMS    AND    MOLECULES. 


weights,  not  forgetting  that  implicit  confidence  cannot  be 
placed  in  the  results  thus  obtained.  It  is  chiefly  useful 
as  an  aid  to  the  other  methods,  and  as  such  it  is  very 
useful.  If  we  examine  those  elements  carefully  which 
form  more  or  less  marked  exceptions  to  the  law,  we  shall 
find  that  they  all  have  small  atomic  weights;  they  belong 
to  the  class  of  bodies  known  as  non-metals;  and  they  are 
either  gases  themselves  or  they  form  compounds  that  are 
gases,  or,  at  least,  volatile.  By  calculation,  the  atomic 
heat  of  hydrogen  has  been  found  to  be  about  2.3  ;  that  of 
oxygen  about  4 ;  that  of  fluorine  about  5 ;  that  of  nitrogen 
about  5.  .Further,  we  have  seen  that  the  atomic  heat  of 
carbon  is  1.1  to  3.1 ;  of  silicon  3.9  to  4.8  ;  of  boron  2.5  to 
2.7 ;  of  sulphur  5.T ;  of  phosphorus  5.4.  Arranging 
these  elements  in  the  order  of  increasing  atomic  weights, 
we  have — 

Atomic  heat. 

2.3 

2.5-2.7 

1.1-3.1 

5    -5.5 

4 

5 

3.9-4.8 

5.4 

5.7 

This  list  includes  all  the  elements  that,  in  their  atomic 
heats,  do  not  comply  with  the  law  of  Dulong  and  Petit. 
It  will  "be  seen  that  the  largest  atomic'weight  represented 
is  that  of  sulphur,  viz.,  32.  Among  the  elements  with 
higher  atomic  weights  no  exceptions  have  as  yet  been 
noticed,  and  hence  the  law  is  looked  upon  as  valid  for 
these  latter.  We  may  then  say  that,  for  elements  with 
high  atomic  weights,  specific  heat  determinations  may  be 
employed  for  the  determination  of  atomic  weights.  But 
these  elements  are  just  the  ones  that  we  cannot  reach  by 
means  of  Avogadro's  rule,  whereas  those  noticed  above 
as  exceptions  can  be;  hence  the  two  methods  supplement 
each  other,  and,  as  far  as  they  may  be  relied  upon,  enable 
us  to  determine  the  atomic  weights  of  all  the  elements. 

Although  we  have  considered  carbon,  silicon,  and  boron 
as  exceptions  to  the  law  of  Dulong  and  Petit,  very  recent 
investigations  show  that,  strictly  speaking,  they  are  not 
exceptions.  The  specific  heats  of  these  elements  increase 


Hydrogen 
Boron 
Carbon 
Nitrogen 
Oxygen 
Fluorine 
Silicon        x 

• 

A. 
1 
11 

12 
14 
16 
19 

28 

Phosphorus 
Sulphur 

31 
32 

SOLID    ELEMENTS    AND    COMPOUNDS.  67 

gradually  from  the  lowest  temperature  to  certain  points, 
when  they  remain  constant  for  any  further  increase  in 
temperature.  The  point  for  carbon  and  boron*  is  in.  the 
neighborhood  of  600°  ;  that  for  silicon  about  200°.  At 
these  temperatures,  and  above  them,  the  elements  have 
the  following  specific  heats :  carbon  0.46 ;  boron  0.50 ; 
silicon  0.205.  The  products  obtained  by  multiplying 
these  figures  by  the  atomic  weights  12,  11,  and  28  are 
5.5,  5.5,  and  5.8 ;  so  that  carbon,  boron,  and  silicon  are 
not  exceptions.  The  law  of  Dulong  and  Petit,  however, 
is  a  little  more  complicated  than  we  have  stated  it  to  be, 
and  should  have  the  following  form: — 

The  specific  heats  of  the  elements  vary  with  the  tem- 
perature; but  for  every  element  there  ia  a  point,  T,  above 
which  the  variations  are  very  slight.  The  product  of  the 
atomic  weight  into  the  constant  value  of  the  specific  heat 
is  nearly  a  constant,  lying  between  5.5  and  6.5. 

It  was  further  found  that  all  the  opaque  modifications 
of  carbon  have  the  same  specific  heat,  and  above  600° 
this  is  the  same  as  that  stated  above,  viz.,  0.46. 

To  account  for  the  variations  in  the  specific  heats  of 
the  elements,  it  has  been  suggested  that  we  cannot  in 
most  cases  determine  the  true  specific  heat.  This  is  only 
that  heat  which  goes  to  increase  the  temperature.  In 
measuring  specific  heats,  we  usually  deal  with  a  complex 
quantity,  viz.,  that  heat  which  raises  the  temperature, 
together  with  that  which  performs  internal  work  and  that 
which  performs  external  work.  In  the  cases  of  solids  and 
liquids,  the  external  work  performed  is  very  small.  The 
internal  work  is  probably  different  in  different  cases,  and 
may  amount  to  considerable.  The  fact,  that  the  specific 
heats  of  so  many  elements  give  with  the  atomic  weights 
of  these  elements  the  same  product,  indicates  that  in  these 
cases  the  external  work,  like  the  specific  heat,1s  inversely 
proportional  to  the  atomic  weights.  It  is  perfectly  evi- 
dent, according  to  this,  that,  if  the  amount  of  internal 
work  varies  in  different  elements,  the  specific  heat  will 

*  According  to  Hampe  (Annalen  der  Chemie,  183,  75),  the 
substance  which  has  always  been  looked  upon  as  the  element 
boron  is  not  the  element.  The  black  crystals  of  Wohler  have 
the  composition  A1B12,  and  the  yellow  crystals  the  composition 
C2A13B48.  In  the  light  of  these  results  it  appears  then  that,  as 
yet,  we  do  not  know  the  specific  heat  of  boron. 


68          DISCUSSION    OP    ATOMS    AND    MOLECULES. 

also  vary  in  such  a  way  as  to  seem  to  conflict  with  the 
law.  It  remains  then  for  the  future  to  show  how  specific 
heat  determinations  can  be  made  which  shall  be  indepen- 
dent of  the  internal  and  external  work.  When  this  can 
be  done,  it  is  probable  that  the  law  of  Dulong  and  Petit 
will  be  found  to  be  a  perfect  law,  without  exceptions  of 
kind. 


ISOMORPHISM  AS  FURNISHING  A  MEANS  FOR  DETERMIN- 
ING ATOMIC  WEIGHTS. — Another  means,  once  considered 
valuable,  for   determining  atomic  weights   is   found   in 
connection  with  isomorphism.     It  has  long  been  known 
that  substances  of  entirely  different   composition  have 
the  same  crystalline  form.     This  was  explained  by  Mit- 
scherlich  (1819)  by  supposing  that  an  equal  number  of 
atoms  in  different  molecules  gave  rise  to  the  same  crys- 
talline form.     A  little  later,  he  proposed  the  following  state- 
ment as  probably  representing  the  law  of  isomorphism  : — 
An  equal  number  of  atoms,  united  in  the  same 
way.,  give  the  same  crystalline  form ;  and  this  crys- 
talline form  is  independent  of  the  chemical  nature 
of  the  atoms,  being  only  dependent  on  their  number 
and  arrangement. 

If  this  law  were  strictly  true,  it  is  plain  that  we  should 
in  many  cases  be  able  to  determine  atomic  weights  by  its 
aid.  A  few  examples  will  illustrate  this  method  : — 

The  two  substances  BaCl2-j-2H,0  and  BaBr.22-fH,O 
are  isomorphous.  We  may  assume,  then,  that  their 
molecules  contain  the  same  number  of  atoms,  and,  if  we 
know  the  atomic  weights  of  the  constituents  of  the  mole- 
cule BaCl.,-|-2H20,  we  can  easily  determine  the  atomic 
weight  of  the  Br  in  the  molecule  BaBr2-f  2H20.  Further, 
the  compounds  CuAgS  and  CuCuS  are  isomorphous. 
Assuming  that  the  molecules  of  each  contain  the  same 
number  of  atoms,  and  knowing  the  atomic  weights  of 
copper  and  sulphur,  we  obtain  very  readily  the  atomic 
weight  of  silver. 

It  cannot  be  denied  that  this  method  has  been  of  ser- 
vice in  the  establishment  of  the  atomic  weights.  Never- 
theless, it  requires  but  a  few  examples  to  prove  that  the 
results  reached  b}r  means  of  it  are  not  perfectly  reliable. 


SOLID    ELEMENTS    AND    COMPOUNDS.  69 

The  salts  BaMn2O8,Na.,S04,  and  Na2Se(54  are  isomorphous, 
and  yet  the  best  methods  for  determining  formulas  show 
that  those  given  are  the  correct  ones.  If  we  were,  in 
these  cases,  to  assume  that  the  number  of  atoms  in  each 
of  the  molecules  is  the  same,  we  would  reach  results  at 
variance  with  those  obtained  by  our  most  reliable  means. 
We  see  thus  that,  if  the  isomorphism  of  salts  is  employed 
as  a  means  for  the  determination  of  atomic  weights,  the 
results  must  be  looked  upon  as  doubtful. 


IY. 

PROPERTIES   OF   THE   ELEMENTS   AS    FUNC- 
TIONS OF  THEIR  ATOMIC  WEIGHTS. 

Natural  Groups  of  Elements. — If  we  examine  the  list 
of  elements  and  their  atomic  weights,  we  find  that  there 
is  a  number  of  well-marked  groups,  indicating  some  con- 
nection between  the  atomic  weights  and  properties  of  the 
elements.  Among  these  may  be  mentioned  chlorine,  bro- 
mine, and  iodine ;  sulphur,  selenium,  and  tellurium ;  lithium, 
sodium,  and  potassium.  Arranging  these  according  to 
their  atomic  weights,  we  have: — 

Cl       35.5  S         32  Li        7 

Br       80  Se       79.5  Na     23 

I       127  Te     128  K      39 

If,  in  each  of  these  groups,  we  add  together  the  atomic 
weights  of  the  first  and  last  elements,  and  divide  this 
sum  by  2,  we  obtain  very  nearly  the  atomic  weights  of 

the  middle  members  of  the  series:  35-5  -  '12'  =  81.25, 

_2  +  128  =  80,  T+39  =  23.  We  see,  also,  that  the  ele- 
ments, whose  atomic  weights  are  thus  closely  connected, 
are  themselves  very  closely  allied  in  their  properties. 
Considerations  of  this  kind  have  led  chemists,  from  time 
to  time,  to  examine  the  atomic  weights  more  closely,  and, 
as  a  result  of  these  examinations,  it  has  been  found  that 
the  connection  above  indicated  is  much  more  general  than 
was  at  first  supposed.  A  number  of  schemes  have  been 
devised  for  the  purpose  of  showing  this  connection  clearly, 
some  of  which  are  certainly  worthy  of  attention.  We  shall 
consider  here  the  schemes  of  D.  Mendelejeff  *  and  Lothar 

*  Zeitschrift  far  Chemie,  1869,  405,  and  Annalen  d.  Ch.  u. 
Pkarm.,  8  Suppl.  133. 


PROPERTIES    OF    THE    ELEMENTS.  71 

Meyer,*  as  they  embody  all  that  is  good  of  other  schemes 
and  are  themselves  comparatively  perfect. 

The  Scheme  of  Mendelejeff.  —  Mendelejeff  first  calls 
attention  to  the  fact  that,  if  all  the  light  elements  with 
atomic  weights  from  7  to  36  are  arranged  in  the  order  of 
their  atomic  weights,  the  following  remarkable  table  is 
obtained  :  — 

Li  =7;     Be=94;     B  =  11    ;     C=12;     N  =  14;    O  =  16;    Fl  =  19. 
Na  =  23;     Mg  =  24    ;    Al  =  27.3;    Si  =  28;    P=31;.S=32;    Cl  =  35.5. 

In  these  tvo  series,  elements  which  we  recognize  as 
similar  come  to  stand  together  as  Li  and  Na,  Mg  and  Be, 
C  and  Si,  O  and  S,  etc.  The  gradual  change  in  the 
properties  of  the  members  of  the  series,  as  we  pass  from 
left  to  right,  is  noticed  particularly,  if  we  consider  the 
compounds  which  the  elements  form.  Thus,  only  the 
four  last  members  combine  with  hydrogen,  yielding  — 

RH4,  RH3,  RHa,  RH. 

The  character  of  these  hydrogen  compounds  also  changes 
gradually,  according  to  the  position  in  the  series.  C1H 
is  a  marked  acid  of  great  stability,  SH2  is  a  weak  acid 
decomposable  by  heat,  PHM  is  not  an  acid,  and  is  less 
stable  than  the  preceding  compounds,  and  this  is  still 
more  true  of  SiH4. 

Considering  the  oxides  of  the  members  of  the  second 
series  we  have  — 


Mg  A,  ALA,  Si  A,  PA,  S  A,  ^  A- 
or  MgO,          or  SiO,,       or  SO3, 

From  left  to  right  in  this  series  the  basic  properties  grow 
weaker  and  the  acid  properties  stronger.  Again,  in  the 
composition  of  the  hydroxides,  the  same  regularity  is 
observed  :  — 

Na(OH),  Mg(OH)  ,  Al(OH),,  Si(OH)41  PO(OH)3, 
SO/OH),,  C10S(OH). 

Another  point  to  be  noted  is  this  :  that,  in  the  series  with 
which  we  are  dealing,  the  metals  are  at  one  end  and  the 
so-called  non-metals  at  the  other,  while  in  the  middle 
those  elements  come  which  are  sometimes  placed  with 

*  Annalen  d.  Ch.  u.  Pharm.,  7  Suppl.  356. 


72          DISCUSSION    OP    ATOMS    AND    MOLECULES. 

the  metals  and  sometimes  with  the  non-metals,  as,  for 
instance,  Si. 

But,  just  as  the  chemical  properties  undergo  gradual 
change  in  the  series  mentioned,  so  also  a  corresponding 
change  is  noticed  in  the  physical  properties.  To  illustrate 
this,  the  specific  gravities  and  the  atomic  volumes  of  the 
members  of  the  second  series  are  given  : — 

Na         Mg  Al  Si  P  S  Cl 

Spec.  gr.   0.97    1.75    2.67    2.49    1.84    2.06     1.33 
Atom.  vol.    24       14       10        11        16        16       27 

Na20     Mg202    A120S    Si,O4      P2O5      SgO,  C1,O7 

Spec.  gr.     2.8      3.7      4.0      2.6      2.7      1.9      ? 
Atom.  vol.     22       22       25       45       55       82       ? 

Another  series  corresponding  to  the  two  already  given 
is  the  following : — 

Ag  =  108;  Cd  =  112;  In  =113;  Sn  =  118;  Sb=122;  Te  =  125?;  1=  127 
Sp.gr.  10.5;        8.6;        7.4;        7.2;         6.7;          6.2;        4.9. 

It  can  be  shown  that  all  the  elements  may  be  arranged 
in  series  similar  to  the  above,  and  thus  a  very  intimate 
connection  between  the  atomic  weights  and  the  properties 
of  the  elements  is  shown  to  exist.  It  will  be  noticed  that 
the  changes  in  the  properties  of  the  elements  are  periodic. 
First  these  properties  change  according  to  the  increasing 
atomic  weights,  then  they  are  repeated  in  a  new  period 
with  the  same  regularity  as  in  the  preceding  series. 
Such  series  as  those  already  mentioned  are  called  small 
periods.  If  H  is  placed  in  the  first  series,  then  Li,  etc., 
come  in  the  second  series,  Na,  etc.,  in  the  third,  etc. 

But  all  the  kjiown  elements  cannot  be  arranged  in  the 
small  periods,  and,  what  is  much  more  important,  the 
corresponding  members  of  the  even  (4,  6,  etc.)  periods, 
or  of  the  uneven  (5,  7,  etc.),  resemble  each  other  more 
closely  than  the  members  of  the  even  periods  resemble 
those  of  the  uneven  periods.  This  may  be  seen  from  the 
following  example: — 

Fourth  period:  K,  Ca,  —  Ti,  V,     Cr,     Mn. 

Fifth          "      :  Cu,  Zn,  —  —  As,   Se,     Br. 

Sixth         "      :  Rb,  Sr,  —  Zr,  Nb,  Mo,  — 

Seventh    "      :  Ag,  Cd,  In,  Sn,  Sb,    Te,    I. 

The  members  of  the  fourth  and  sixth  periods  resemble 
each  other  more  closely  than  they  resemble  the  members 


PROPERTIES    OF    THE    ELEMENTS  73 

of  the  fifth  or  seventh  periods;  and  the  members  of  the 
fifth  and  seventh  periods  resemble  each  other  closely. 
The  last  members  of  the  even  periods  resemble  in  many 
respects  the  first  members  of  the  succeeding  uneven 
series.  Thus  Cr  and  Mn  in  their  basic  oxides  are  similar 
to  Cu  and  Zn.  On  the  other  hand,  between  the  last 
members  of  the  uneven  periods  and  the  first  members  of 
the  succeeding  even  periods,  there  are  very  marked 
differences,  as,  for  instance,  between  Br  and  Rb.  Further, 
between  the  last  members  of  the  even  periods  and  the 
first  members  of  the  uneven  periods,  all  those  elements 
which  cannot  be  arranged  in  the  small  periods  would, 
according  to  their  atomic  weights  and  properties,  natu- 
rally come.  Thus  between  Cr  and  Mn,  on  the  one  hand, 
and  Cu  and  Zn,  on  the  other,  Fe,  Co,  and  Ni  would 
come ;  the  following  series  being  thus  formed  : — 

Cr  =  52;  Mn  =  55;  Fe  =  56;  Co  =  59;  Ni  =  59; 
Cu  =  63;  Zn  =  65. 

As  Fe,  Co,  Ni  follow  the  fourth  period,  so  Ru,  Rh,  Pd 
follow  the  sixth  period,  Os,  Ir,  Pt  follow  the  tenth  period. 
Two  small  periods  (an  even  and  an  uneven),  together 
with  an  intermediate  series  of  the  elements  just  men- 
tioned, form  a  large  period.  As  the  intermediate  mem- 
bers mentioned  correspond  to  none  of  the  seven  small 
periods,  they  form  an  independent  eighth  group:  — 

Fe  =  56 ;  Ni  =  59 ;  Co  =  59. 
Ru=rl04;  Rh  =  104;  Pd  =  106. 
Os  =193?;  Ir  =  195?;  Pt  =  197. 

The  members  of  this  group  resemble  each  other  in  the 
same  way  as  the  corresponding  members  of  the  even 
periods,  as,  for  instance,  Y,  Nb,  Ta,  or  Cr,  Mo,  W,  and 
others. 

The  two  following  tables  of  Mendelejeff  show  clearly 
the  relations  described.  In  the  first,  the  elements  with 
their  atomic  weights  are  arranged  in  large  periods ;  in 
the  second,  they  are  arranged  in  groups  and  series  in 
such  a  manner  as  to  distinctly  indicate  the  differences  in 
the  even  and  uneven  periods. 
7 


74          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

TABLE  I. 


. 

K     =  39 

Rb   =85 

Cs    =133 

— 



Ca   =  40 

Sr     =87 

Ba    =137 

— 

— 

— 

?Yt  =88? 

?Di  =138?  Er    =178? 

— 

Ti     =48? 

Zr     =90 

Ce    =140?  ?La  =180? 

Th    =  231 

V      =51 

Nb   =94 

—         Ta    =182 

— 

Cr    =  52 

Mo   =96 

— 

W    =184 

U      =  240 

Ma  =  55 

— 

— 

— 

— 

Fe    =  56 

Ru   =104 

—          Os    =195? 



Typical 

Co    =  09 

Rh  =104 

—         jlr     =197 



elements. 

r-  •  , 

Ni    =  59 

Pd    =106 

—         Ft    =198? 

— 

H=l|Li     =7     Na=23 

Ca    =63 

Ag    =108 

— 

Au  =199? 

— 

Be    =  9.4Mg  =  24 

Zn    =  65 

Cd    —112 

—          Hg  =200 

— 

B      =  11    Al   =273 

- 

In     =113 

—          Tl     =201 

— 

C      =12   Si    =28 

— 

Sn    =118 

—          Pb   =207 

— 

N      «.  14  IP    »  31 

As    =  75 

Sb    =122 

—         ;Bi    =208 

— 

0      =  16   S      =32 

S      —  78 

Te    =125? 

— 

- 

— 

F      =  19   Cl   =  35.5  Br    =  80 

I       =127 

— 

— 

— 

It  is  not  necessary  here  to  point  out  all  the  properties 
of  the  elements  which  can  be  shown  to  vary  in  harmony 
with  the  changes  of  the  atomic  weights.  What  has  already 
been  said  will  suffice  to  indicate  the  principle  involved  in 
the  construction  of  the  tables  of  Mendelejeff.  Close  study 
of  these  tables  would  undoubtedly  show  that  they  contain 
some  imperfections  and  apparent  contradictions ;  still,  these 
are  not  numerous  enough  or  serious  enough  to  materially 
interfere  with  the  value  of  the  tables.  It  is  evident  that 
the  first  condition  for  the  construction  of  such  tables  is 
the  correct  determination  of  all  the  atomic  weights.  We 
have  seen  with  what  difficulty  this  determination  is  often 
attended,  and  how  doubtful  some  of  the  results  obtained 
are.  When  all  the  atomic  weights  shall  have  been  accu- 
rately determined,  and  when  all  the  properties,  both  phy- 
sical and  chemical,  of  the  elements  are  known,  then  a 
taltle  constructed  on  the  principle  involved  in  the  con- 
struction of  the  above  tables  will,  in  all  probability,  show 


PROPEUTIES    OF    THE     ELEMENTS. 


75 


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76          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

a  perfect  accordance  between  atomic  weights  and  proper- 
ties. Perhaps  then,  too,  the  law  of  variation  being  known, 
it  may  be  possible  to  propose  a  satisfactory  theory  in 
regard  to  the  nature  and  conduct  of  atoms.  Such  a 
theory  will  be  the  legitimate  result  of  the  scientific  chem- 
ical investigations  now  being  carried  on.  The  atomic 
theory  of  Dalton,  as  at  present  accepted,  will  eventually 
prove  to  be  but  a  part  of  a  more  comprehensive  atomic 
theor}',  which  latter  will  be  the  theory  of  chemistry,  cor- 
responding to  the  theory  of  heat  and  the  theory  of  elec- 
tricity in  the  broad  domain  of  physics. 

Mendelejeff  proposes  to  use  the  periodic  law,  as  he 
calls  the  simple  rule  governing  the  variations  in  the 
atomic  weights  and  properties  of  the  elements  in  his 
tables,  for  the  purpose  of  determining  the  properties  of 
undiscovered  elements.  In  Table  II.  it  will  be  seen  that, 
in  the  fourth  series,  a  member  is  wanting  between  calcium 
and  titanium.  The  atomic  weight  of  this  element  would 
be  about  44,  and  its  properties  can  also  be  very  nearly 
foretold  from  its  position.  It  would  somewhat  resemble 
boron ;  its  oxide  would  have  the  composition  R20S,  and 
its  properties  would  bear  the  same  relation  to  A1203  that 
CaO  bears  to  MgO,  or  Ti02  to  Si02 ;  consequently  it 
must  be  a  more  energetic  base  than  A12O8  and  would 
resemble  it  in  corresponding  compounds.  Its  sulphate 
would  not  be  as  easily  soluble  as  aluminium  sulphate, 
because  calcium  sulphate  is  more  difficultly  soluble  than 
magnesium  sulphate.  Thus,  throughout  the  whole  list  of 
properties,  such  comparisons  are  made,  and  the  unknown 
element  is  more  accurately  described  than  some  of  those 
which  have  been  known  for  a  long  time. 

Such  speculations  are,  without  doubt,  very  attractive, 
but  we  must  not  forget  that  the  tables,  which  are  used  as 
their  foundation,  are  more  or  less  imperfect,  and  hence 
the  conclusions  drawn  must  necessarily  be  doubtful.  On 
the  other  hand,  the  time  will  come  when  such  speculations 
can  be  indulged  in  without  risk  of  reaching  doubtful  re- 
sults. The  approach  of  this  time  will  be  hastened  by 
just  such  efforts  as  those  of  Mendelejeff  to  discover  the 
law  governing  the  connection  between  the  atomic  weights 
and  the  properties  of  the  elements." 


PROPERTIES    OF    THE    ELEMENTS. 


77 


Lothar  Meyer' s  Arrangement  of  the  Elements. — Another 
arrangement  of  the  elements  showing  the  connection  be- 
tween the  atomic  weights  and  the  properties  of  the  ele- 
ments is  that  of  Lothar  Meyer,  already  alluded  to.  His 
table  follows — 


I. 
II. 

III. 

IV. 
V. 
VI. 

VII. 
VIII. 
IX. 

B 
11.0 

Al 
273 

? 

47? 

? 

70? 

C 
11.97 

Si 

28 

Ti 

48 

? 

72  ? 

N 
14.01 

P 

30.96 

V 

51.2 

As 

0 
15.96 

S 
31.98 

Cr 
52.4 

Se 

F 
19.1 

Cl 
33.37 

Mn 
54.8 

Fe 
55.9 

Co 
58.6 

H 
1 

Ni 
58  6 

Li 

7.01 

Na 
2299 

K 

39.  Oi 

Cu 

Be 
9.3 

Mg 
23.94 

Ca 
39.90 

Zn 

? 

88? 

In 
1134 

? 
173? 

Tl 

202.7 

Zr 

90 

SQ 
117.8 

? 
178? 

Pb 

206  4 

74.9 

Nb 
94 

Sb 
122 

Ta 
182 

Bi 

207.5 

78 

Mo 
95.6 

Te 

128 

W 

184 

Br 
79.75 

? 

98? 

I 
126.53 

? 
186? 

Ru 
1035 

Os 
198.6 

Rh 
104.1 

Ir 

1967 

Pd 

106.2 

Pt 

196.7 

63  3 

Rb 
85.2 

Ag 
107.66 

Cs 
1327 

Au 
196.2 

64.9 

Sr 

87.2 

Cd 
111.6 

Ba 
1368 

Hg 
199  8 

This  table  contains  all  the  elements  whose  atomic 
weights  have  been  determined  with  any  degree  of  cer- 
tainty. The  series  here  represented  resemble  those  in 
Mendelejeff  's  tables.  In  some  respects,  the  arrangement 
is  more  imperfect  than  that  already  considered.  Meyer 
further  points  out  the  connection  between  the  atomic 
weights  and  the  following  properties  of  the  elements: 
specific  gravity  in  the  solid  condition,  as  shown  by  a 
comparison  of  the  atomic  volumes;  metallic  ductility, 
fusibility  and  volatility ;  crystalline  form;  influence  upon 
the  refraction  of  light ;  specific  heat;  conducting  power 
for  heat  and  electricity.  The  connection  is  not  always 
very  clearly  discernible,  but  careful  examination  and  the 
exclusion  of  sources  of  error  as  far  as  possible  show  that 
the  connection  is  an  actual  one.  It  remains  for  the  future 
to  establish  the  close  connection  which  exists,  but  which 
is  thus  far  mostly  hidden. 

7* 


78          DISCUSSION    OP    ATOMS    AND    MOLECULES. 

The  position  of  the  elements  in  the  electro-chemical 
series  is  undoubtedly  dependent  on  the  position  in  the 
series  of  atomic  weights,  but  it  is  such  an  exceedingly 
difficult  thing  to  determine  the  proper  order  of  the  mem- 
bers in  the  electro-chemical  series,  that  the  connection 
between  the  two  series  is  not  plainly  recognizable. 


Y. 

VALENCE  OR  ATOMICITY  OF  ELEMENTS. 

Definition. — The  means  for  the  determination  of  the 
molecular  formulas  of  compounds  have  already  been  con- 
sidered, and  it  has  been  shown  that  these  formulas  can 
be  determined  with  a  considerable  degree  of  certainty  by 
the  aid  of  Avogadro's  hypothesis.  If  we  now  examine 
the  formulas  as  determined,  certain  new  characteristics 
of  the  elements  will  present  themselves.  Let  us  take  a 
few  examples  among  the  h}7drogen  compounds, 
i.  ir.  in.  iv. 

C1H,        OH,,        Ntfs,         CH4, 

BrH,        SH2,         AsH3,       SiH4. 

IH,          SeH ,       SbH3, 

F1H,        TeH"2,       PHa, 

We  see  here  that  Cl,  Br,  I,  and  Fl  each  combine  with 
H  in  the  proportion  of  atom  for  atom;  0,  S,  Se,  and  Te 
combine  with  H  in  the  proportion  of  two  atoms  of  hydro- 
gen for  one  atom  of  the  other  element ;  in  the  compounds 
with  N,  As,  Sb,  and  P,  three  atoms  of  hydrogen  are  in 
combination  with  one  atom  of  the  other  element:  and, 
lastly,  four  atoms  of  hydrogen  are  in  combination  with 
one  atom  of  C  or  Si.  The  elements  here  mentioned  are 
representatives  of  four  great  classes.  If  we  consider 
the  members  of  the  first  class,  we  shall  find  that  the 
compounds  which  they  form  with  each  other  are  of  the 
simplest  kind.  Indeed,  they  combine  with  each  other 
only  in  one  proportion,  forming  thus  but  one  kind  of 
compounds.  If  we  take,  for  instance,  hydrogen  and  chlo- 
rine and  allow  them  to  combine  under  the  most  varied 
conditions,  the  result  is  always  hydrochloric  acid,  and 
this  always  contains  35.5  parts  by  weight  of  chlorine  to 
1  part  by  weight  of  hydrogen.  The  same  is  true  of 
other  members  of  the  class,  as  bromine,  potassium,  sodium, 


80          DISCUSSION    OP    ATOMS    AND    MOLECULES. 

etc.  If,  on  the  other  hand,  we  consider  the  members  of 
any  of  the  other  classes,  we  shall  find  that  a  greater 
variety  presents  itself  in  their  combinations,  not  only 
with  each  other,  but  with  the  elements  of  the  first  class. 
Ox3'gen  combines  with  hydrogen  in  two  proportions, 
forming  water  and  hydrogen  peroxide;  nitrogen  combines 
with  oxygen  in  five  proportions,  forming  nitrous  oxide, 
nitric  oxide,  nitrogen  dioxide,  nitrous  anhydride,  and 
nitric  anhydride;  carbon  combines  with  hydrogen  in  a 
very  large  number  of  proportions,  forming  several  series 
of  hydrocarbons.  In  the  foregoing,  we  have  an  indica- 
tion of  some  property  of  the  elements  which  we  have  not, 
thus  far  considered.  In  studying  the  elements  now  with 
reference  to  this  new  property,  our  first  duty  would 
plainly  be  to  studj^  all  the  compounds  of  the  elements  in 
order  to  get  as  broad  a  foundation  of  facts  as  possible. 
We  would  thus  finally  be  enabled  to  draw  the  following 
conclusion : — 

Some  elements  combine  with  each  other  only  in  one 
proportion;  others  combine  with  each  other  and  with 
other  elements  in  more  than  one  proportion. 

This  conclusion  involves  no  hypothesis,  but  is  the  legiti- 
mate outgrowth  of  the  known  facts.  The  distinction  made 
between  the  two  classes  of  elements  is  fundamental. 

But  there  must  be  some  reason  for  the  difference. 
Where  shall  we  seek  for  it?  If  we  accept  the  atomic 
theory  of  Dalton,  we  must  seek  for  the  proximate  causes 
of  phenomena  presented  to  us  by  masses  of  elements  or 
their  compounds  in  the  atoms  composing  these  masses. 
Here,  then,  we  are  to  find  in  the  atoms  themselves  the 
proximate  cause  of  the  new  property  of  elements  which 
we  are  considering.  As,  however,  we  can  learn  nothing 
of  atoms  directly,  but  only  of  masses  of  atoms,  it  is 
evident  that  this  cause  cannot  be  discovered,  but  must 
finally  be  imagined  just  as  the  atom  itself  is  still  only 
imagined  to  exist.  In  other  words,  our  ideas  of  atoms 
must  become  enlarged  in  such  a  way  as  to  account  for 
the  new  property,  or  a  subordinate  hypothesis  must  be 
formed  to  supplement  the  atomic  theory.  Before  forming 
this  hypothesis  let  us  see  whether  we  can  learn  any  tiling- 
more  in  regard  to  the  new  property  than  we  have  yet 
learned. 

We  have  seen  above  that,  on  examining  the  formulas 


VALENCE    OR    ATOMICITY    OP    ELEMENTS  81 

of  chemical  compounds  and  comparing  them  with  each 
other,  the  molecules  of  some  of  the  compounds  contain 
only  one  atom  of  hydrogen  to  each  atom  of  the  other 
element;  in  the  molecules  of  other  compounds  we  find 
two  atoms  of  hydrogen  to  one  atom  of  the  other  con- 
stituent; in  others  three,  and  in  still  others  four  atoms 
of  hydrogen  combined  with  one  atom  of  some  other  ele- 
ment. In  thus  stating  the  number  of  atoms  present  in  the 
molecules  we,  of  course,  have  left  the  domain  of  facts  and 
have  already  entered  that  of  hypothesis;  still,  the  hypo- 
thesis involved  in  a  molecular  chemical  formula  which 
has  been  determined  by  the  aid  of  all  means  at  our  com- 
mand is  one  which  we  are  justified  in  employing,  and  we 
accordingly  add  the  knowledge  which  we  gain  by  means 
of  this  hypothesis  to  that  which  we  possess  from  the 
simple  study  of  facts,  and  which  is  represented  in  our 
conclusion  above  drawn.  Taking  into  consideration  the 
sum  total  of  our  knowledge,  as  thus  far  stated,  the  sim- 
plest hypothesis  which  it  is  possible  to  form  concerning 
the  cause  which  we  are  striving  to  find  would  be  the  fol- 
lowing: Every  atom  of  an  element  has  an  inherent  power 
of  holding  in  combination  a  certain  number  of  other 
atoms,  this  number  being  dependent  upon  the  combining 
power  of  the  atoms  held  in  combination.  The  simplest 
atoms  would  represent  the  unit  of  this  power,  and  we 
would  distinguish  between  these  simplest  or  unit-atoms, 
and  such  as  have  the  power  of  holding  in  combination 
two,  three,  four,  or  more  unit-atoms. 

Name  of  the  New  Property. — The  property  of  the  ele- 
ments which  we  are  studying  has,  in  accordance  with  the 
simple  hypothesis  just  given,  been  termed  atomicity, 
quantivalence,  or  only  valence.*  The  elements  which 

*  In  regard  to  the  choice  between  the  three  expressions  given, 
it  may  be  said  that  the  word  valence  seems  to  be  less  objection- 
able than  the  others  which  have  been  used,  because  it  is  the  sim- 
plest and,  at  the  same  time,  it  expresses  all  that  we  desire  to 
express  with  reference  to  the  property  which  it  designates. 
Again,  the  word  atomicity  has  been  used  in  another  sense,  and 
hence  its  use  might  lead  to  confusion,  some  authors  employ  ing- 
it  in  its  first  sense,  others  in  the  new  and  entirely  different  sense. 
By  a  monatomic,  diatomic,  etc.  element  is  sometimes  meant  an 
element  whose  molecule  consists  of  one,  two,  etc.  atoms.  As  it 
is  necessary  to  have  words  to  express  this  latter  sense,  it  seems 


82          DISCUSSION    OF    ATOMS    AND    MOLECULES 

consist  of  the  unit-atoms  are  hence  called  monatomic  or 
univalent;  those  consisting  of  atoms  which  have  twice 
the  combining  power  of  the  unit-atoms  are  called  dia- 
tomic or  bivalent;  and  in  a  similar  manner  we  have 
triatomic  or  trivalent,  tetratomic  or  quadrivalent,  etc. 
elements.  Further,  the  elements  are  called  respectively 
monads,  dyads,  triads,  tetrads,  pentads,  hexuds,  etc.  Ac- 
cordingly, of  the  elements  in  the  brief  table,  given  at  the 
beginning  of  this  article  (p.  79),  all  those  in  the  first 
line  are  univalent;  oxygen,  sulphur,  selenium,  and  tellu- 
rium are  bivalent;  nitrogen,  arsenic,  antimony,  and 
phosphorus  are  trivalent;  and  carbon  and  silicon  are 
quadrivalent. 

Distinction  between  Valence  and  Affinity. — The  pro- 
perty of  valence  must  not  be  confounded  with  that  of 
affinity.  By  affinit}^  is  usuall}'  meant  the  force  with 
which  one  atom  attracts  another  or  others.  Valence  has 
apparently  no  connection  with  this  force.  An  element  may, 
in  general  terms,  have  a  strong  affinity  for  other  elements, 
and  yet  be  univalent.  Another  may  possess  but  a  weak 
affinity  and  be  quadrivalent.  Thus,  chlorine  has  a  strong 
affinity  for  hydrogen,  the  two  combine  with  great  energy, 
yet  they  are  both  univalent  elements.  While  carbon  does 
not  combine  with  chlorine  with  nearly  as  great  energy  as 
hydrogen  does,  it  nevertheless  is  quadrivalent;  its  atom 
is  capable  of  holding  in  combination  four  atoms  of  hydro- 
gen. The  two  properties,  valence  and  affinity,  are  pos- 
sessed by  every  atom  and  exhibit  themselves  every  time 
that  atoms  act  upon  each  other,  the  latter  determining 
the  energy  of  the  action,  the  former,  the  complexity  of 
the  resulting  molecule. 

Methods  for  determining  the  Valence  of  the  Elements. — 
The  foundations  upon  which  the  conception  of  valence 
rests,  and  the  conception  itself  being  thus  explained,  let 
us  inquire  how  we  can  determine  the  valence  of  the 
individual  elements.  We  have  recognized  certain  cha- 

desirable  to  leave  the  word  atomicity  and  its  adjective-deriva- 
tives, monatomic,  diatomic,  etc.,  to  serve  this  purpose,  while  we 
adopt  the  expression  valence  with  the  derivatives  univalent, 
bivalent,  trivalent,  etc.,  for  the  purpose  of  designating  the  pro- 
perty under  discussion. 


VALENCE    OR    ATOMICITY    OP    ELEMENTS.  83 

racteristics  of  the  so-called  nnivalent  elements,  and  have 
seen  that  hydrogen  belongs  to  this  class.  If  then  we  are 
certain  that  hydrogen  is  univalent,  we  may  employ  it  as 
a  means  of  measuring  the  valence  of  other  elements. 
The  first  general  rules  to  guide  us  in  the  measurements 
would  be — 

1.  Those  elements  which  combine  with  hydrogen   in 
the  proportion  of  1  atom  to  1  atom  are  univalent.     Such, 
for  instance,  are  chlorine,  bromine,  etc. 

2.  Those  elements  which  combine  with  hydrogen  in 
the  proportion  of  1   atom  to  2  atoms  of  hydrogen  are 
bivalent.     Such,  for  instance,  are  oxygen,  sulphur,  etc. 

3.  Those  elements  which  combine  with  hydrogen  in 
the  proportion  of  1  atom  to  3  atoms  of  hydrogen  are 
trivalent.     Such,  for  instance,  are  nitrogen,  phosphorus, 
etc. 

4.  Those  elements  which  combine  with  hydrogen  in 
the  proportion  of  1  atom  to  4"  atoms  of  hydrogen  are 
quadrivalent.     Such,  for  instance,  are  carbon,  silicon,  etc. 

But  only  comparatively  a  small  number  of  the  elements 
combine  with  hydrogen  alone,  so  that  this  method  must 
necessarily  be  of  limited  application.  It  is  plain,  however, 
that,  if  our  conception  of  valence,  as  above  explained,  is 
correct,  it  is  not  necessary  that  we  should  employ  hydro- 
gen as  our  standard  in  measuring  it.  Any  other  univalent 
element  may  answer  the  same  purpose.  Having  then 
first  determined  by  means  of  hydrogen  that  chlorine  and 
bromine  are  univalent,  we  may  employ  either  of  these  as 
standards  in  the  same  manner  as  we  employed  hydrogen 
above.  This  would  enable  us  to  extend  our  measure- 
ments much  further,  and,  eventually,  to  take  in  all  the 
elements.  In  all  cases  in  which  the  molecular  formulas 
of  the  chlorine  or  hydrogen  compounds  of  the  elements 
can  be  determined  according  to  the  principles  of  Avoga- 
dro's  hypothesis,  the  determination  of  the  valence  is, 
according  to  the  rules  given,  a  simple  matter. 

To  illustrate  the  application  of  the  method,  let  us  take 
an  example.  Suppose  we  wish  to  determine  the  valence 
of  oxygen.  Oxygen  forms  two  compounds  with  hydrogen. 
We  analyze  both  of  these  compounds,  and  find  that  water 
contains  the  smallest  proportion  of  oxygen  to  hydrogen. 
We  assume,  therefore,  that  in  the  molecule  of  water  there 
is  but  one  atom  of  oxygen ;  and  we  consequently  take 


84          DISCUSSION    OF    ATOMS    AND    MOLECULES. 

this  compound  for  the  purpose  of  making  our  determina- 
tion. The  molecular  weight  is  found  to  be  18  ;  and  this, 
taken  in  conjunction  with  the  results  of  the  analyses, 
shows  us  that  16  parts  of  oxygen  are  combined  with  2 
parts  of  hydrogen.  The  atomic  weight  of  hydrogen  is 
already  known  to  be  1.  Hence  one  atom  of  oxygen  is 
combined  with  two  atoms  of  hydrogen;  and  hence, 
further,  oxygen  is  a  bivalent  element. 

Insufficiency  of  the  Hypothesis.  —  In  attempting  to 
subdivide  the  elements  into  classes  according  to  their 
valence,  difficulties  are  met  with  ;  and  these  are  so  great 
that  the  hypothesis  as  already  given  proves  inadequate, 
and  must  undergo  change  in  order  to  be  in  harmony  with 
the  facts.  .The  conception  of  valence  with  which  we  have 
been  dealing  is  the  simplest  which  is  held.  It  is,  too,  the 
first  form  in  which  it  was  brought  to  light.  For  these 
reasons  it  has  been  explained  in  full.  It  is  now  neces- 
sary, however,  to  show  in  what  respects-  it  is  inadequate, 
and  to  show  what  changes  it  has  undergone. 

The  original  form  of  our  hypothesis,  as  above  stated, 
was:  E very  atom  has  an  inherent  power  of  holding  in 
combination  a  certain  number  of  other  atoms,  this  number 
being  dependent  upon  the  combining  power  of  the  atoms 
held  in  combination. 

According  to  this,  phosphorus,  which  combines  with 
hydrogen,  forming  the  compound  PH3,  ought,  in  com- 
bining with  chlorine,  to  form  the  compound  PCL,  and 
there  stop.  Nitrogen,  which  forms  the  compound  NH8, 
ought  to  show  itself  as  a  trivalent  element  wherever  it  is 
met  with.  In  the  case  of  phosphorus,  however,  we  have 
not  only  the  chlorine  compound  PCla,  but  another,  PCL; 
in  the  case  of  nitrogen,  too,  we  have  not  only  a  whole 
series  of  compounds  in  which  it  plays  the  part  of  a 
trivalent  element,  but  also  a  very  large  series  in  which  it 
just  as  surely  plays  the  part  of  a  quinquivalent  element. 
Thus  we  have,  on  the  one  hand,  ammonia,  NH3, 

in  it        i  in  n      i 

hydroxylnmine,  NH2  (OH),  nitrous  acid,  NO  (OH),  etc., 
in  which  nitrogen  is  trivalent ;  while,  on  the  other  hand,  we 

V     IV       I  V      IV  I 

have  ammonium  chloride,  NH4C1,  nitric  acid,  N02(OH), 
and  the  whole  list  of  nitrates  and  ammonium  salts  in 


VALENCE    OR    ATOMICITY    OF    ELEMENTS.  85 

which  nitrogen  is  quinquivalent.  A  number  of  other 
instances  could  be  given,  showing  that  one  and  the  same 
element  exhibits  different  combining  powers  in  different 
compounds,  so  that,  if  we  attempted  to  classify  the  ele- 
ments according  to  the  valence,  we  would  place  the  same 
element  now  with  the  trivalent  and  now  with  the  quin- 
quivalent; another  would  find  its  place  with  the  bivalent 
and  again  with  the  quadrivalent,  according  as  one  or 
another  compound  of  these  elements  is  taken  as  the 
means  of  judging  of  the  valence.  Plainly,  then,  the  con- 
ception of  valence  needs  enlargement,  for,  as  above  stated, 
it  has  not  a  true  foundation  of  facts. 

Atomic  and  Molecular  Compounds. — The  difficulty 
just  indicated  was  early  recognized,  and  an  attempt  was 
made  to  surmount  it  by  introducing  another  hypothesis 
in  regard  to  the  nature  of  chemical  compounds.  Accord- 
ing to  this  new  hypothesis,  there  are  two  kinds  of  chemical 
compounds,  which  are  called  atomic  and  molecular.  In 
the  former  of  these,  we  have  the  true  chemical  compounds, 
in  the  sense  in  which  we  have  understood  that  expression 
from  the  beginning.  In  these,  the  atoms  exhibit  all  the 
properties  which  we  have  thus  far  recognized  as  belonging 
to  them — including  valence.  By  virtue  of  these  proper- 
ties, the  compounds  have  their  existence.  In  the  mole- 
cular compounds,  on  the  other  hand,  a  new  force  is  sup- 
posed to  act,  this  force  being  distinct  from  the  interatomic 
force,  and  acting  in  a  peculiar  way  between  molecules. 
The  molecules  are  supposed  to  be  first  formed  by  means 
of  chemism,  etc.,  and,  when  these  have  been  formed,  all 
that  can  be  effected  by  valence  has  been  effected.  But 
now  it  is  further  supposed  that  the  molecules  thus  formed 
have  an  attractive  power  and  combining  power  of  their 
own,  by  virtue  of  which  the  molecular  compounds  are 
formed.  The  most  common  examples  of  molecular  com- 
pounds are  salts  containing  water  of  crystallization. 
These  are  formed  by  virtue  of  the  attraction  of  the  mole- 
cules of  the  salt  for  the  molecules  of  the  water.  But, 
according  to  the  propounder  of  this  hypothesis,  we  have 
further  examples  of  molecular  compounds  in  phosphorus 
chloride,  PCL,  and  in  ammonium  chloride,  NH4C1.  In  the 
former  of  these,  a  molecule  of  PCI.,  a  true  atomic  com- 
pound, holds  in  combination  a  molecule  of  chlorine  (C12), 
8 


86  DISCUSSION    OF    ATOMS    AND    MOLECULES. 

also  an  atomic  compound.  In  the  latter,  a  molecule  of 
NH3,  an  atomic  compound,  holds  in  combination  a  mole- 
cule of  hydrochloric  acid,  also  an  atomic  compound. 

Foundation  for  the  Distinction  between  Atomic  and 
Molecular  Compounds. — Of  course,  in  order  that  such 
an  hypothesis  as  that  under  consideration  should  be  at 
all  permissible,  it  must  be  shown  that  there  are  differ- 
ences between  those  compounds  which  are  called  molec- 
ular and  those  which  are  called  atomic.  To  a  certain 
extent  this  is  possible.  In  the  case  of  salts  containing 
water  of  crystallization,  there  can,  at  least  usually,  be 
no  difficulty  in  recognizing  that  the  force  holding  together 
the  salt  and  the  water  is  of  a  different  nature  from  that 
which  holds  the  constituents  of  the  salt  together,  or  that 
which  holds  the  constituents  of  the  water  together.  It 
is  only  necessary  to  heat  the  compounds  gently  in  order 
to  overcome  the  attraction  and  cause  the  breaking  up  of 
the  complex  molecule ;  in  some  cases,  indeed,  the  attrac- 
tion is  so  weak  that  it  is  only  necessary  to  expose  the 
compound  to  the  air,  when  the  water  passes  off  in  the  form 
of  vapor,  leaving  the  molecules  of  the  salt  intact.  This 
weakness  of  the  union  is  then  a  principal  characteristic  of 
molecular  combination. 

Now,  if  we  examine  the  compounds  above  referred  to, 
viz.,  PC15  and  NH4C1,  we  find  that  they  possess  this  cha- 
racteristic. It  has  been  shown,  when  considering  the 
cases  of  anomalous  specific  gravities  of  vapors,  that, 
when  PC15  and  NH4C1  are  heated  to  a  sufficient  degree 
to  convert  them  into  vapor,  they  are  broken  up  into 
PC13  and  Cl,,  and  NH3  and  HC1,  just  as  the  crystal- 
lized salts  lose  their  water  of  crystallization  by  being 
heated.  So,  also,  in  a  number  of  other  cases,  it  can  be 
shown  to  be  true  that  compounds,  which  we  must  con- 
sider as  molecular  in  order  to  explain  their  existence  and 
yet  retain  the  hypothesis  of  valence,  as  above  stated, 
break  up  under  the  influence  of  heat  into  simpler 
molecules. 

We  thus  see  that  there  is  some  foundation  for  assuming 
the  existence  of  molecular  compounds  as  distinct  from 
atomic  compounds.  But  how  does  this  help  us  in  sur- 
mounting the  difficult3'  already  met  with  in  attempting 


VALENCE    OR    ATOMICITY    OF    ELEMENTS.  87 

to  apply  the  principle  of  the  hypothesis  of  valence  for 
the  purpose  of  classifying  the  elements  ? 

Use  of  the  Distinction. — It  is  plain  that  atomic  com- 
pounds would  be  the  only  ones  which  we  could  employ 
for  the  purpose  of  determining  the  valence  of  atoms. 
Thus,  the  only  chlorine  compound  of  phosphorus  which 
we  could  employ  for  the  purpose  of  determining  the 
valence  of  phosphorus  is  PClb.  The  other  compound, 
PCL,  the  existence  of  which  would  seem  to  indicate  that 
phosphorus  is  quinquivalent  as  well  as  trivalent,  being  a 
molecular  compound,  is  formed  independent  of  the  val- 
ence of  phosphorus  and  does  not  at  all  interfere  with  the 
acceptance  of  the  original  hypothesis  of  valence.  So, 
too,  in  all  similar  cases.  Nitrogen  is  really  trivalent, 
but,  owing  to  the  formation  of  molecular  compounds,  it 
appears  oftentimes  to  be  quinquivalent.  By  a  generous 
application  of  this  principle,  there  is  no  difficulty  in  ac- 
counting for  the  anomalous  cases,  and  our  hypothesis  of 
valence  may  still  be  retained,  unless  it  can  be  shown  that 
there  are  facts,  not  yet  considered,  which  do  not  har- 
monize with  it. 

Difficulties  met  with. — One  difficulty  immediately  pre- 
sents itself.  Although  the  examples  above  given  show 
that  there  are  compounds  which  seem  to  differ  from  true 
chemical  compounds  to  a  sufficient  extent  to  justify  us 
in  assuming  that  they  belong  to  another  class  of  com- 
pounds, still,  there  are  cases  in  which  there  is  no  ground 
whatever  for  making  this  assumption,  and  which,  never- 
theless, show  plainly  that  one  and  the  same  element  may 
be  at  one  time  trivalent,  at  another  quinquivalent,  unless 
we  make  the  above  assumption  without  ground.  The 
compound  POCl;i,  for  instance,  is  not  decomposed  when 
converted  into  the  form  of  vapor,  and  we  have  just  as 

much  right  to  consider  it  a  true  chemical  compound  as 

ii  MI 

we  have  to  consider  PC13  as  such.  But,  in  POCl.^,  phos- 
phorus is  quinquivalent,  while  in  PC13  it  is  trivalent. 
Evidently,  here  our  only  ground  for  considering  POOL,  a 
molecular  compound  is  the  fact  that  its  existence  cannot 
be  explained  by  the  original  hypothesis  of  valence.  This 
is  dangerous  reasoning,  and,  if  we  follow  it,  we  shall 


88  DISCUSSION    OP    ATOMS    AND    MOLECULES. 

soon  be  in  serious  difficulties.  We  saw  above  that  all 
the  nitrates  and  ammonium  salts  can  only  be  explained 
on  the  supposition  that  the  nitrogen  in  them  is  quinqui- 
valent, unless  we  suppose  them  to  be  molecular  com- 
pounds. Plainly,  it  would  be  next  to  absurd  to  indulge 
in  the  latter  supposition,  as  both  the  nitrates  and  the 
ammonium  salts  have  all  the  characteristics  of  true  chem- 
ical compounds,  and,  if  we  can  assume  that  they  are  only 
molecular  compounds,  in  order  to  suit  our  purpose,  then 
we  are  at  liberty  to  make  the  same  assumption  in  regard 
to  almost  any  compound  in  the  field  of  chemistry. 

Experiments  showing  that  Nitrogen  may  be  both  Tri- 
valent  and  Quinquivalent. — .Again,  an  experiment  has 
been  recently  performed  which  appears  to  show  that 
ammonium  chloride,  NH4C1,  and  analogous  compounds 
of  nitrogen,  are  true  atomic  compounds.  If  NH4C1  is  a 
molecular  compound,  then,  as  was  explained  above,  two 
forces  are  concerned  in  the  formation  of  its  molecule. 

1st.  A  force  holding  together  the  nitrogen  atom  and 
three  hydrogen  atoms  forming  the  molecule  }NTH3;  and 
the  hydrogen  atom  and  chlorine  atom  forming  the  mole- 
cule HC1. 

2d.  A  force  holding  together  the  molecule  NH3  and 
the  molecule  HC1. 

If  these  two  forces  are  distinct  in  character,  the  re- 
sulting molecule  might  be  represented  by  the  formula 
(NH3  -f-  HC1).  Suppose  now  we  add  together  two  other 
molecules  such  that,  taken  together,  they  are  the  same  in 
number  and  quantity  with  those  contained  in  the  com- 
pound (NH3  -f-  HC1).  Then  the  resulting  compound 
ought  not  to  be  identical  with  that  obtained  in  the  former 
case.  If  these  new  molecules  .are,  for  instance,  NH2C1 
and  H2,  then  the  compound  will  be  (NH.,C1  +  HH)  and 
this  should  not  be  identical  with  (NH3  -f-  HC1),  although 
its  composition  is  exactly  the  same. 

This  principle  has  been  tested,  not,  indeed,  with  the 
molecules  employed  in  the  above  explanation,  but  with 
others  analogous  to  them.  Instead  of  NHS,  the  analogous 
compound,  N(CHa)3,  was  taken  and  this  was  united  with 
(C2H5)1.  Thus,  a  compound  was  obtained  which,  if  it  be 
molecular,  may  be  represented  by  the  formula  (N(CIL)3-{- 
CaH&l).  Again  the  compound  N(CH3)aCidH5  was  taken, 


VALENCE    OR    ATOMICITY    OP    ELEMENTS.  89 

and  this  was  united  with  CH.J,  yielding  a  compound 
which,  as  in  the  former  case,  may  be  represented  by  the 
formula  (N(CH3),C,H5  +  (CH,)I).  Now,  these  two  new 
compounds  ought  not  to  be  identical,  if  they  are  molecular 
and  not  atomic.  On  comparing  them,  however,  they  were 
found  to  be  in  every  respect  identical. 

From  this  experiment  it  is  concluded  that  the  com- 
pounds studied  are  atomic  compounds,  and  that  in  them 
nitrogen  is  quinquivalent.  Each  group  (CH,),  (C.2H_), 
and  the  element  I  being  held  by  the  same  kind  of  force, 
the  identity  of  the  resulting  compounds  is  readily  under- 
stood. We  have  in  each — 

CH, 


As  was  stated  in  a  previous  chapter,  the  compound 
PCI.  can  be  converted  into  the  form  of  vapor  under  cer- 
tain circumstances,  viz.,  in  the  presence  of  the  vapor  PCla. 
From  this  it  is  concluded  that  the  compound  PCL  is  a 
true  chemical  or  atomic  compound ;  and  hence,  further, 
that  the  phosphorus  atom  contained  in  it  is  quinqui- 
valent. 

Th,e  Distinction  between  Atomic  and  Molecular  Com- 
pounds unnecessary  as  far  as  the  Hypothesis  of  Valence 
is  concerned. — We  recognize  thus  that  nitrogen  and 
phosphorus  act  in  some  compounds  as  trivalent,  in  other 
compounds  as  quinquivalent,  elements.  If  we  acknowledge 
this,  however,  then  there  is  no  necessity  for  assuming  the 
existence  of  molecular  compounds  for  the  purpose  of 
explaining  anomalies  in*  the  valence  of  elements.  It  is 
very  probable,  indeed,  that,  in  the  so-called  double  com- 
pounds in  which  we  have  two  or  more  salts  combined 
with  each  other,  as  well  as  in  the  salts  containing  water 
of  crystallization,  we  have  true  examples  of  molecular 
compounds,  in  the  sense  in  which  this  expression  has 
been  used  in  the  present  article;  but  it  is  evident  that, 
as  soon  as  we  admit  the  possibility  of  one  and  the  same 
element  being  either  univalent  or  bivalent,  it  is  no  longer 
necessary  to  assume  the  existence  of  these  molecular 

8* 


90  DISCUSSION    OF    ATOMS    AND    MOLECULES. 

compounds.  We  hence  leave -the  study  of  these  peculiar 
and  interesting  compounds  for  the  future,  and  continue 
our  discussion  of  valence. 

Saturated  and  Unsaturated  Compounds. — As  soon  as 
the  quinquivalence  of  nitrogen  was  established,  a  new 
explanation  was  offered  concerning  the  nature  of  nitrogen 
compounds.  Only  those  compounds  in  which  the  nitrogen 
is  quinquivalent  were  looked  upon  as  complete.  Those 
in  which  the  nitrogen  is  trivalent  were  looked  upon  as 
incomplete.  For  the  expressions  complete  and  incomplete, 
the  expressions  saturated  and  unsaturated  were  employed. 
The  atom  of  nitrogen  having  the  power  to  hold  in  com- 
bination five  nnivalent  elements  or  groups  is  saturated 
when  all  of  its  powers  are  employed,  as  in  the  compound 
NH401 ;  it  is  unsaturated  when  only  a  part  of  its  powers 
are  employed,  as  in  the  compound  NH3.  The  power  of 
the  nitrogen  atom  was  expressed  by  saying  that  it  pos- 
sesses five  affinities.  In  the  saturated  compound  all  of 
these  affinities  are  employed,  whereas  in  the  unsaturated 
compound  only  a  part  of  them  are  employed. 

To  explain  the  fact  that  ammonium  chloride,  NH4C1, 
is  readily  decomposed  by  heat,  yielding  NH3  and  HC1,  it 
was  further  supposed  that  of  the  five  affinities  of  the 
nitrogen  atom  two  are  weaker  than  the  other  three. 
Hence,  in  a  saturated  nitrogen  compound,  two  atoms  or 
groups  are  held  less  strongly  than  the  other  three,  and 
are  given  off  more  readily.  This  same  explanation  would 
account  for  the  decomposition  of  phosphorus  chloride, 
PCL,  into  PCl3and  C12,  and  the  other  similar  decomposi- 
tions to  which  reference  has  already  been  made.  The 
experiment  described  above,  however,  which  proved  the 
identity  of  the  compounds-^(N(CH3)3  -f  C2H5I)  and 
N(CH3)2(C,H5) -f-CH3I— proved  also  that  the  affinities 
of  the  nitrogen  atom  are  all  of  the  same  kind,  and  hence 
we  cannot  admit  that  two  of  the  affinities  are  weaker  than 
the  other  three.  While,  further,  phosphorus  chloride, 
PC15,  is  readily  decomposed  by  heat,  in  accordance  with 
the  supposition  that  two  of  the  affinities  of  the  phosphorus 
atom  are  weaker  than  the  other  three,  yet,  on  the  other 
hand,  the  compound  POC13  gives  no  evidence  of  this 
difference  of  the  affinities.  1 1  can  also  easily  be  shown 
that  it  is  not  necessary  to  assume  this  difference  in  order 


VALENCE    OR    ATOMICITY    OP    ELEMENTS.  91 

to  explain  the  decomposition  of  phosphorus  chloride, 
PCL,  and  ammonium  chloride,  NH4C1.  We  may,  for 
instance,  suppose  that  the  five  affinities  of  the  nitrogen 
atom,  or  the  phosphorus  atom,  are  all  exactly  equal  in 
power  at  the  outset.  Should  three  of  these  affinities, 
however,  become  saturated,  it  seems  possible  that  the 
presence  of  the  saturating  atoms  or  groups  may  influence 
the  power  of  the  remaining  unsaturated  affinities  in  such 
a  way  as  to  make  them  weaker  than  they  were  at  first. 

With  the  evidence  before  us,  it  seems  that  we  are 
justified  in  abandoning  the  view  that  the  affinities  of  the 
nitrogen  atom  and  similar  pentavalent  atoms  differ  from 
each  other  in  the  strength  of  the  attraction  which  they 
exert  towards  other  atoms.  It  remains  then  yet  to 
explain,  by  the  aid  of  some  other  hypothesis,  the  ready 
decomposition  of  ammonium  chloride  and  phosphorus 
chloride. 

Double  Union. — To  account  for  the  existence  of  un- 
saturated compounds,  some  chemists  have  supposed  that 
two  affinities  of  the  same  atom  might  in  some  way  act 
upon  each  other,  causing  saturation.  The  compound  in 
which  such  a  combination  exists  would  then  be  a  com- 
plete compound,  not  possessing  free  affinities.  In  them, 
however,  the  mutual  saturation  of  like  affinities  could  be 
easily  overcome,  and  then  other  atoms  could  be  held  in 
combination.  This  was  supposed  to  be  rendered  probable 
by  the  fact  that  the  number  of  affinities  which  are  con- 
sidered as  free  in  unsaturated  compounds  is  always,  with 
very  few  exceptions,  an  even  number.  This  would 
necessarily  be  the  case  if  the  above  assumption  were 
true.  The  idea  of  double  union  has  undoubtedly  been  of 
service  in  some  cases,  as,  for  instance,  in  the  theory  of 
the  so-called  aromatic  compounds;  but  it  remains  still  to 
be  shown  whether  there  is  a  sufficient  basis  of  facts  for  it 
to  rest  upon. 

Variable  Valence. — We  have  already  seen  that  nitro- 
gen and  phosphorus  act  in  some  compounds  as  though 
they  were  trivalent;  in  others,  as  though  they  were  quin- 
quivalent. On  examining  the  compounds  of  other  ele- 
ments, as  we  have  examinnd  those  of  nitrogen  and  phos- 
phorus, we  should  find  that  some  of  these  also  appear  at 


92  DISCUSSION    OF    ATOMS    AND    MOLECULES. 

times  to  have  one  valence,  and  at  other  times  another 
valence.  As  a  consequence  of  such  observations,  the 
hypothesis  of  valence  as  first  stated  in  its  simplest  form 
was  changed,  and  the  change  has  been  accepted  by  some. 
Instead  of  supposing  the  valence  of  an  atom  to  be  a  con- 
stant property  determined  by  the  nature  of  the  atom,  just 
as  the  weight  and  many  other  properties  are  necessarily 
connected  with  the  atom  and  are  constant,  it  was  supposed 
that  the  valence  could  change  according  to  circumstances. 
These  circumstances  might  be  of  various  kinds,  but  pro- 
minent among  them  was  the  temperature.  Accordingly, 
in  some  of  their  compounds,  nitrogen  and  phosphorus 
are  trivalent,  and  in  others  quinquivalent.  Inasmuch  as 
the  ammonium  compounds,  in  which  the  nitrogen  is  quin- 
quivalent, are  decomposed  by  heat,  yielding  ammonia,  in 
which  the  nitrogen  is  trivalent,  and  as,  further,  phosphorus 
chloride,  in  which  the  phosphorus  is  quinquivalent,  is  de- 
composed by. heat,  yielding  phosphorus  trichloride,  in 
which  the  phosphorus  is  trivalent,  the  conclusion  was 
drawn  that,  at  lower  temperatures,  the  valence  of  these 
elements  is  greater,  and  that  the  valence  decreases  with 
an  increase  of  temperature. 

The  change  thus  made  in  the  hypothesis  of  valence  has 
led  to  a  much  more  comprehensive  change  affecting  the 
valence  of  every  element.  Not  only  could  the  valence  of 
some  few  elements  vary  according  to  circumstances,  but 
now  the  valence  of  all  elements  is  variable.  The  same 
element  may  be  univalent,  trivalent,  quinquivalent,  hep- 
tavalent,  etc.  Another  may  be  bivalent,  quadrivalent, 
hexavalent,  etc.  All  the  elements  are  divided  into  two 
classes,  called  artiads  and  perissads.  The  members  of 
the  former  class  may  have  any  valence  represented  by  an 
even  number,  as  2,  4,  6,  8,  etc.  Those  of  the  latter  class 
may  have  any  valence  represented  by  an  uneven  number 
as  1,  3,  5,  T,  etc.  There  is,  to  be  sure,  a  prevailing  ten- 
dency on  the  part  of  each  element  to  act  with  some  par- 
ticular valence,  but,  nevertheless,  as  occasion  demands, 
new  powers  may  be  called  into  requisition,  or  some  of 
those  which  usually  exhibit  themselves  may  disappear. 

Objections  to  the  Idea  of  Variable  Valence. — The  ob- 
jection which  we  have  to  this  view  is  the  following:  We 
conceive  the  valence  of  an  element  to  be  a  very  important 


VALENCE    OB    ATOMICITY    OF    ELEMENTS.  93 

property.  Hence,  the  difference  between  univalence,  tri- 
valence,  and  quinquivalence  is  a  great  difference,  and  the 
change  from  one  to  the  other  is  a  great  and  important 
change.  To  say  that  an  atom  lias  now  the  power  of  hold- 
ing in  combination  only  one  univalent  atom,  and  that,  in 
so  doing,  all  its  power  is  employed ;  and,  again,  to  say 
that  the  same  atom  has  three,  five,  or  even  seven  times 
this  power,  presupposes  very  peculiar  notions  concerning 
the  nature  of  force.  Where  shall  we  find  any  analogy  for 
this  startling  kind  of  metamorphosis  ?  This  power  of  the 
atoms,  it  must  be  remembered,  is  actual  power,  it  is  tan- 
gible, it  is  a  form  of  energy.*  Do,  then,  the  atoms  origi- 
nate and  annihilate  energy  at  will?  Can  they  multiply 
or  divide  the  energy  which  they  possess  in  one  compound 
by  five  or  seven,  so  as  to  be  correspondingly  stronger  or 
weaker  in  another  compound?  If  we  accept  this  view, 
we  must  ascribe  to  the  atoms  themselves  creative  power 
and  the  power  to  annihilate  energy.  This  we  prefer  not 
to  do,  and  hence,  we  do  not  accept  the  hypothesis  of  vari- 
able valence,  in  the  sense  in  which  that  expression  has 
been  used  in  this  article. 

Wurtz's  View.  —  Wurtz  has  recently  emplo3'ed  the 
term  valence  in  a  sense  differing  entirely  from  that  in 
which  we  have  thus  far  understood  it,  and  in  which  it  has 
been  used  by  most  writers.  By  it  he  means  the  power 
which  an  atom  actually  exhibits  in  any  given  compound, 
not  the  absolute  power  of  the  atom  to  hold  other  atoms 
in  combination.  He  says:  tl  We  think  that  the  important 
thing  is  not  to  fix  the  atomicity  which  each  element  pos- 
sesses absolutely,  but  that  which  it  manifests  in  a  given 
compound."  He  then  points  out  the  difficulties  in  the 
way  of  determining  the  absolute  atomicity  of  an  element, 
and  abandons  the  attempt  to  accomplish  the  determina- 
tions. Further,  he  calls  attention  to  the  fact  that  the  force 
with  which  one  atom  attracts  another  depends  upon  the 
properties  of  both  atoms,  and  cannot  be  measured  abso- 
lutely by  the  force  of  one  of  the  atoms.  So  also,  he  con- 

*  The  potential  energy  of  a  free  atom  must  be  dependent  upon 
two  things,  viz  ,  the  affinity  of  the  atom  for  other  atoms,  and 
the  valence  of  the  atom,  determining  the  number  of  atoms  which 
are  attracted  by  means  of  the  affinity. 


94  DISCUSSION    OF    ATOMS    AND    MOLECULES. 

ceives  that  an  atom  may  be  trivalent  towards  one  kind  of 
atoms  and  quinquivalent  towards  another.  He  says,  in 
this  connection:  "The  property  which  phosphorus  pos- 
sesses of  uniting  with  five  atoms  of  chlorine  depends, 
without  doubt,  upon  a  particular  condition  of  the  atoms, 
their  form,  structure,  volume,  movements.  This  con- 
dition, which  is  unknown  to  us,  is  invariable  for  phos- 
phorus, and  if  one  atom  of  phosphorus  can  attract  five 
atoms  of  chlorine,  while  it  can  only  unite  with  three 
atoms  of  hydrogen;  if  it  is  quinquivalent  as  regards 
chlorine,  and  trivalent  as  regards  hydrogen,  we  ought  to 
seek  for  the  reason  of  these  differences,  not  only  in  the 
atoms  of  phosphorus,  but  also  further  in  those  of  chlo- 
rine and  of  hydrogen." 

The  proposition,  then,  is  to  throw  aside  the  idea  of 
valence,  as  it  has  been  defined,  and  to  substitute  for  it  a 
variable  idea.  The  valence  of  an  atom  is  thus  not  a 
fixed  property  of  the  atom  determining  the  nature  of  the 
compound  which  this  atom  forms  with  others,  but,  these 
compounds  having  been  formed  by  virtue  of  unknown 
properties  of  atoms,  the  valence  is  that  particular  com- 
bining power  which  the  atom  happens  to  exhibit.  The 
difference  between  these  two  conceptions  of  valence  are 
as  great  as  that  between  the  conception  of  atomic  weight 
and  equivalent.  It  will  be  remembered  that,  owing  to 
the  difficulties  at  first  encountered  in  determining  atomic 
weights,  it  was  proposed  to  substitute  the  equivalent  for 
the  atomic  weight,  and  that  the  atomic  weight  was,  for  a 
time,  abandoned.  The  subsequent  development  of  the 
science  showed  that  this  step  was  a  backward  step,  and 
that  the  first  conception  of  Dalton  of  the  atomic  weight 
was  the  true  one.  Equivalents  have,  in  turn,  been  aban- 
doned, and  the  definite  atomic  weights  have  again  taken 
their  place. 

We  believe  that  the  proposition  of  Wiirtz  to  abandon 
the  definite  idea  of  valence  is  analogous  to  the  original 
proposition  of  Wollaston  to  abandon  the  definite  idea  of 
the  atomic  weight.  There  are  undoubtedly  strong 
reasons  for  the  step  proposed  by  Wurtz,  as  there  was  for 
the  step  proposed  by  Wollaston.  The  difficulties  in 
the  way  of  determining  the  absolute  valence  of  an  ele- 
ment are  fully  as  great  as  they  are  represented  to  be  by 
Wiirtz.  Nevertheless  we  are  not  prepared  to  follow  his 


VALENCE    OR    ATOMICITY    OF    ELEMENTS.  95 

suggestion  to  abandon  the  definite  idea  of  valence.  We 
believe  this  valence  to  be  a  fixed  and  important  property 
of  every  atom,  the  study  of  which  will  in  time  lead  to 
valuable  results.  When  investigation  shall  have  pro- 
ceeded far  enough  to  enable  us  better  to  understand  this 
property  than  we  do  at  present,  we  shall  probably  find, 
as  we  have  held  throughout  this  discussion,  that  the 
valence  of  the  atom  determines  in  every  case  the  com- 
plexity of  the  molecule ;  and  that,  knowing  the  valence 
and  nature  of  the  atoms  entering  into  combination,  we 
shall  be  able  to  foretell  the  composition  of  the  molecule. 
Because  we  cannot  do  this  at  present,  shall  we  abandon 
the  idea  completely  ?  We  do  not  think  it  advisable  to 
do  so.  That  which  Wiirtz  calls  atomicity  we  would  call 
apparent  valence,  thus  distinguishing  it  from  the  true 
valence.  The  distinction  having  thus  been  made,  let  us 
point  out  more  clearly  the  differences  between  apparent 
and  true  valence. 

True  Valence. — This  has  already  been  defined.  By 
true  valence  is  meant  that  property  of  an  atom  by  virtue 
of  which  it  has  the  power  to  hold  in  combination  a  certain 
number  of  other  atoms.  It  is  an  invariable  property  for 
the  same  atom  under  the  same  conditions.  It  may  be 
exerted  to  the  full  extent  in  a  given  compound,  or  may 
not  be.  According  as  it  is,  or  is  not,  thus  exerted  to  the 
full  extent,  the  compounds  which  it  forms  are  saturated 
or  unsaturated.  In  an  unsaturated  compound  there  are 
free  affinities,  but  these  are  not  as  strong  as  the  affinities 
of  the  free  atom,  for  these  must  change  with  the  entrance 
into  a  molecule  of  new  atoms.  For  instance,  the  affinities 
of  an  atom  of  phosphorus  are  at  first  equal  in  strength, 
and  in  every  other  respect.  But,  as  soon  as  a  part  of 
these  affinities  are  employed  in  holding  atoms  in  place, 
it  seems  very  probable  that  the  presence  of  these  new 
atoms  would  influence  the  nature  of  the  whole  molecule ; 
and,  if  there  are  any  unemplo3^ed  affinities  present,  these 
too  would  differ  from  the  unemployed  affinities  of  the 
free  atom.  In  some  instances,  this  influence  may  be 
comparatively  strong,  and  the  unemployed  affinities  may 
thus  be  rendered  very  weak,  so  weak,  indeed,  that  they 
are  hardly  recognizable.  In  other  instances,  this  influence 


96  DISCUSSION    OF    ATOMS    AND    MOLECULES. 

ma}7  be  less  marked,  and  then  we  would  have  more  active 
unsaturated  bodies,  that  is  to  say,  such  as  would  readily 
take  up  additional  atoms. 

Methods  for  determining  True  Valence. — In  regard  to 
the  means  at  our  command  for  measuring  the  valence  of 
elements,  we  must  confess  to  great  weakness.  It  is 
impossible  at  present  to  measure  the  true  valence  of  all 
elements  with  any  degree  of  accuracy.  We  adopted 
hydrogen  above  as  the  standard  for  making  the  measure- 
ments. It  still  seems  to  us  proper  to  do  so.  It  does  not 
follow,  however,  that  the  results  obtained  through  the 
aid  of  the  hydrogen  compounds  are  necessarily  the  true 
ones.  In  determining  atomic  weights,  we  do  not  only 
take  into  consideration  one  particular  set  of  compounds, 
but  all  compounds  of  an  element,  and  then  adopt  for  the 
determination  that  particular  compound  which  contains 
the  smallest  proportion  of  the  element.  So,  in  determin- 
ing the  valence,  it  is  right  to  take  into  consideration  all 
the  compounds  of  an  element,  and  then  to  adopt  that  one 
which  contains  the  largest  proportion  of  the  measuring 
element. 

We  believe,  further,  that  there  are  other  elements, 
besides  hydrogen,  and  other  atomic  groups  which  are 
univalent  in  the  same  way  that  Ir^drogen  is  univalent. 
Such,  for  instance,  are  chlorine  and  bromine,  methyl 
(CH3)  and  ethyl  (C2H,).  These  elements  and  groups 
may  be  used  for  the  purpose  of  determining  the  valence 
of  other  elements,  just  as  hydrogen  is  used.  It  may  be 
that  the  results  obtained  by  means  of  them  may  differ 
from  those  obtained  by  means  of  hydrogen.  This  is  not 
necessarily  fatal  to  the  method,  though  such  a  disagree- 
ment of  results  might  incline  us  to  think,  as  Wiirtz 
thinks,  that  the  valence  of  an  element  is  not  only  depen- 
dent on  the  nature  of  its  own  atoms,  but  also  upon  the 
nature  of  the  atoms  with  which  it  is  combined.  If  this 
supposition  of  Wiirtz  is  correct,  however,  if,  as  he  says, 
phosphorus  is  quinquivalent  towards  chlorine,  how  can 
we  explain  the  existence  of  the  compound  PC1{  without 
supposing  it  to  be  an  unsaturated  compound  ?  What 
difference  shall  we  understand  to  exist  between  PH3  and 

pcy 


VALENCE    OR    ATOMICITY    OF    ELEMENTS.  97 

Apparent  Valence. — The  apparent  valence  of  an  ele- 
ment is  that  which  Wiirtz  calls  the  atomicity.  It  may  or 
may  not  be  identical  with  the  true  valence.  In  every 
compound  in  which  the  apparent  valence  of  an  element 
is  not  its  true  valence,  there  are  free  affinities.  These 
may  or  may  not  be  masked  through  the  influence  of  the 
atoms  or  groups  already  present  in  the  molecule.  In 
many  cases,  the  apparent  valence  of  an  element  in  any 
given  compound  will  be  determined  as  soon  as  the  formula 
of  the  compound  is  determined.  In  cases  where  it  is 
possible  to  ascribe  to  an  element  more  than  one  valence, 
it  is  better  to  select  the  prevailing  valence  which  it 
exhibits  in  its  other  compounds,  unless  there  are  good 
reasons  for  selecting  some  other.  We  ought  always, 
however,  to  take  the  simplest  view  permissible. 

With  the  conception  of  apparent  valence  just  explained, 
and  with  the  limitations  recommended,  it  will  be  found 
that  no  element  exhibits  more  than  two  different  powers 
of  combination,  the  majority,  only  one.  It  seems  pro- 
bable that  in  those  cases  in  which  an  element  exhibits 
only  one  valence,  this  may  be  the  true  valence.  Whether 
this  be  so,  is  a  question  still  to  be  answered.  Our  know- 
ledge of  true  valence  is  so  limited  that,  for  the  present, 
we  propose  to  call  the  valence  observed  the  apparent- 
valence,  leaving  the  proper  development  of  the  subject 
of  true  valence  for  the  future. 

Conclusions — As  a  result  of  the  above  discussion  we 
are  prepared  to  make  the  following  statements,  which  we 
believe  to  be  in  accordance  with  the  facts,  and  the  simplest 
which  can  be  made  in  the  present  state  of  our  knowledge. 

I.  Valence  is  a  fixed  property  of  elements,  which  may 
be  fully  exerted  or  not. 

II.  We  do  not  possess  satisfactory  means  for  deter- 
mining the  true  valence  of  elements. 

III.  We  can  determine  the  apparent  valence  of  elements. 
IY.  The  apparent  valence   may  vary   within   narrow 

limits,  but  rarely,  if  ever,  does  one  element  exhibit  more 
than  two  powers. 

In   the  following  table  the  apparent  valence  of  each 
element,  as  determined  by  a  variety  of  observations,  is 
indicated: — 
9 


98 

DISCUSSION    OF 

I. 

Univalent  Elements. 

Hydrogen, 

H 

Chlorine, 

Cl 

Bromine, 

Br 

Iodine, 

I 

Fluorine, 
Potassium, 

Fl 
K 

Sodium, 

Na 

Caesium, 

Cs 

Rubidium, 

Rb 

Lithium, 

Li 

Silver, 

Ag 

II. 

Bivalent  Elements. 

Oxygen, 

0 

Sulphur, 

S 

Selenium, 

Se 

Tellurium, 

Te 

Barium, 

Ba 

Strontium, 

Sr 

Calcium, 

Ca 

Magnesium, 

Mg 

Beryllium, 

Be 

Zinc, 

Zn 

Cadmium, 

Cd 

Copper, 

Cu 

Mercury, 

Hg 

Yttrium, 

Y 

Erbium, 

Er 

% 

Cerium, 

Ce 

Lanthanium, 

La 

Didyniium, 

Di 

III 

Trivalent  Elements. 

Boron, 

B 

Bismuth, 

Bi 

Gold, 

Au 

Indium, 

In 

IV. 

Bi-and  Trivalent  Elements. 

Nickel, 

Ni 

Cobalt, 

Co 

Iron, 

Fe 

Manganese, 

Mn 

Chromium, 

Cr 

Aluminum, 

Al 

Uranium, 

U 

V. 

Bi-  and  Quadrivalent  Ele- 

ments. 

Carbon, 

C 

Tin, 

Sn 

Lead, 

Pb 

Platinum, 

Pt 

Iridium, 

Ir 

Palladium, 

Pd 

Osmium, 

Os 

Rhodium, 

Rh 

Ruthenium, 

Ru 

VI. 

Quadrivalent 

Elements. 

Silicon, 

Si 

Titanium, 

Ti 

Zirconium, 

Zr 

Thorium, 

Th 

The  above  table  is,  no  doubt,  exceedingly  imperfect. 
Investigations,  at  present  being  carried  on,  are  tending  to 
perfect  it.  Through  a  knowledge  of  the  apparent  valence 
of  elements,  we  shall  eventually  arrive  at  a  knowledge  of 
the  true  valence,  and  hence  we  attach  importance  to  the 
study  of  this  peculiar  property.  We  are  far  from  under- 
standing it,  so  that  any  discussion  of  it  must  necessarily 
be  very  imperfect.  In  the  above,  the  attempt  has  been 
made  to  show  exactly  where  we  stand  with  reference  to 
this  propertj^,  to  show  that  some  of  the  hypotheses  which 
have  been  proposed  corcerning  this  property  have  not 
foundation  enough  to  warrant  us  in  accepting  them,  and 
to  reduce  the  hypotheses  back  to  the  simplest  form. 
This  is  all  we  can  do  at  present,  and  with  this  we  close 
our  discussion. 


PART  SECOND. 

CONSTITUTION  OR  STRUCTURE  OF 
CHEMICAL  COMPOUNDS. 


I. 

GENERAL  CONSIDERATIONS. 

DEFINITION  OF  CONSTITUTION,  ETC. 

Definition,  etc. — In  considering  the  subject  of  valence, 
we  stated  that  chemical  compounds  are  formed  by  virtue 
of  the  mutual  attraction  of  the  free  affinities  of  atoms 
upon  each  other;  that  the  valence  of  atoms  determines 
the  complexity  of  the  molecules  into  which  they  enter. 
When  the  compounds  have  once  been  formed,  the  affini- 
ties called  into  action  are  no  longer  free;  they  are 
saturated.  Upon  this  mutual  neutralization  or  saturation 
of  free  affinities  are  based  our  ideas  in  regard  to  the 
constitution  or  structure  of  chemical  compounds.  The 
simplest  illustrations  of  chemical  constitution  are  found, 
of  course,  among  the  compounds  which  univalent  elements 
form  with  each  other.  Whatever  conceptions  we  hold  in 
regard  to  the  constitution  of  these  simplest  compounds, 
we  must,  in  general,  hold  the  same  conceptions  in  regard 
to  the  constitution  of  the  most  complicated  compounds. 
Let  us  then  briefly  consider  one  simple  compound,  and 
explain  definitely  what  we  mean  by  its  constitution. 

An  atom  of  hydrogen  in  a  free  condition  is  a  particle 
of  matter  having  definite  weight.  This  particle  of  matter 
has  the  power  of  attracting  and  combining  with  other 
atoms.  Further,  the  number  of  other  atoms  which  it  can 
hold  in  combination  is  limited  to  one  of  the  simplest  kind, 
as,  for  instance,  one  atom  of  hydrogen.  Now,  if  two 
atoms  of  hydrogen  be  brought  in  contact  with  each  other, 
the  powers  which  they  possess  are  called  into  play,  and, 


100  CHEMICAL    COMPOUNDS. 

as  a  consequence,  the  two  unite.  As  soon  as  the  union 
has  taken  place,  the  powers  cease  to  be  free,  they  are 
performing  their  appropriate  functions.  In  the  place  of 
the  atoms,  we  have  now  a  molecule  of  hydrogen  which 
has  not  of  itself  any  direct  attractive  power,  enabling  it 
directly  to  form  chemical  compounds.  We  are  not  able 
to  state  in  what  manner  the  free  affinities  of  the  two 
hydrogen  atoms  have  satisfied  each  other.  It  is  not  at 
all  likely,  however,  that  a  firm  union  between  the  two 
atoms  takes  place,  in  such  a  way  as  to  render  the  parts 
of  the  molecule  immovable  with  reference  to  each  other. 
Much  more  likely  is  it  that,  after  the  union,  the  atoms 
perform  some  kind  of  motion  with  reference  to  each  other, 
according  to  the  laws  of  atomic  motion  yet  to  be  dis- 
covered. For  our  present  purpose,  it  is  sufficient  to 
know  that,  in  whatever  manner  the  union  takes  place, 
the  chemical  activity  of  the  atoms  ceases  in  consequence. 
The  union  then  may  be  represented  by  the  formula  H.H, 
or  by  H — H  ;  the  point,  in  the  one  case,  and  the  line,  in 
the  other,  merely  indicating  the  fact  of  union,  without 
explaining  the  manner  of  the  union.  By  the  constitution 
of  the  molecule  of  hydrogen,  then,  we  mean  the  arrange- 
ment of  the  atoms  in  it.  Not  the  actual  arrangement  of 
these  atoms  in  space,  for  in  regard  to  this  we  know 
absolutely  nothing. 

In  general  terms,  then,  by  the  constitution  of  a  mole- 
cule, we  mean  the  arrangement  of  the  atoms  composing 
it.  We  have  determined  the  constitution  of  a  chemical 
compound  when  we  have  determined  which  atoms  are 
united  with  each  other  in  its  molecule.  We  express  the 
constitution  by  means  of  a  graphic  formula,  indicating 
the  union  of  the  atoms  present,  as,  for  instance,  the 
following  are  constitutional  or  graphic  formulas  : — 

/H  .H 

H.O.H     or      O<       ;  N-H     or     N— H; 

H  *H  \H 

Water.  Ammonia. 

H.    .H  H 


C  or  ^  ^TJ 

H-  -H 

Marsh-gas  (methane). 


GENERAL    CONSIDERATIONS.  101 

H 


H.    .H 

N  // 

H.    .H  or  N— H   . 

Cl 


Ammoniam  chloride. 

Possible  forms  of  Combination. — The  simplest  forms 
of  combination  found  among  the  elements  are  those  in 
which  one  nnivalent  element  is  combined  with  another 
univalent  element.  We  have  instances  of  this  kind  of 
combination  in  the  molecules  of  the  univalent  elements 
themselves,  as  H— H,  Cl— Cl,  etc. ;  and,  further,  in  such 
compounds  as  the  following  :— 

H—C1,          H— Br,          K— 01,          NaBr,      etc. 

Hydrochloric         Hydrobromic  Potassium  Sodium 

acid.  acid.  chloride.  bromide. 

We  next  have  those  compounds  in  which  one  bivalent  is 
combined  with  two  univalent  elements,  as,  for  instance:  — 

H  _O— H,      K— 0— H,     Cl— 0— H,     Na— 0— Na,  etc. 

Water.  Potassium  Hypochlorous  Sodium  oxide, 

hydroxide.  acid. 

One  bivalent  may  be  united  with  one  other  bivalent 
element,  as  in  the  following : — 

Ba=0,  Ca=O,  Mg  =  O,      etc. 

Barium  oxide.  Calcium  oxide.  Magaesium  oxide. 

One  triad  may  be  in  combination  with  three  monads, 
as: — 


etc. 


One  triad  may  be  in  combination  with  one  dyad  and  a 
monad:  — 


One  triad  may  be  combined  with  one  triad  :  — 
B^N;    etc. 

Nitrogeu-boron. 

9* 


II 

/C1 

K 

/C1 

N—  H  , 

P—  Cl  , 

N/H  , 

As—  Cl  , 

\H 

\C1 

VH 

\C1 

AmmoDia. 

Phosphorus 

Potassium 

Arseuious 

chloride. 

amide. 

chloride. 

V  S  S/|  g  J 


102  CHEMICAL    COMPOUNDS. 

One  tetrad  may  be  combined  with  four  monads: — 

H  H  H  H 

C/H  .  C/H   .  C/H   .  0/C1 

XH  XCl  XC1  XC1 

Methane.  Monochlormethane.         Dichlormethane.     Trichlormethane. 

Cl 

;      etc. 

xei  ^01 

Silicon-chloroform.  Lead  chloride. 

One  tetrad  may  be  combined  with  one  dyad  and  two 
monads : — 


C—H  ;  C—  H  ;         etc. 

\H  \H 

Formic  aldehyde.  Thioforrnic  aldehyde. 

One  tetrad  may  be  combined  with  two  dyads:  — 

.o  ,o  s 


/. 


;  Si          ;  C         ;        etc. 


Carbon  dioxide  Silicon  dioxide.        Carbon  disulphide. 

One  tetrad  may  be  combined  with  one  triad  and  one 
monad  :  — 

cfW;      etc. 
H 

Hydrocyinic  acid. 

One  tetrad  may  be  combined  with  another  tetrad:  — 
0=0. 

Probably  the  molecule  of  carbon. 

One  pentad  may  be  combined  with  five  monads:  — 
Cl  /H  H 

f!l  /  TT  /  TT 


P—  Cl;  N—  H  ;'  N—  H  ;      etc. 

\C1  \\H  \\H 

XC1  XC1  \Br 

Phosphorus  chloride.  Ammonium  chloride.     Ammonium  bromide. 


GENERAL    CONSIDERATIONS.  103 

One  pentad  ma}7  be  combined  with  one  dyad  and  three 
monads:  — 


01  '  \  Cl  '  Br  '       etc' 

XC1  XC1  XBr 

Phosphorus  oxichloride.    Phosphorus  sulphochloride.     Phosphorus  oxibromide. 

One  pentad  may  be  combined  with  two  dyads  and  one 
monad  :  — 


One  pentad  may  be  combined  with  one  triad  and  two 
monads  :  — 


One  pentad  may  be  combined  with  one  triad  and  one 
dyad  :  — 


One  pentad  may  be  combined  with  one  tetrad  and  one 
monad  :  — 


In  regard  to  the  four  cases  last  mentioned,  it  will  be 
noticed  that  no  examples  are  given.  No  combinations  of 
these  kinds  are  really  known.  As  far  as  we  know,  how- 
ever, they  are  theoretically  possible,  and  perhaps,  in  the 
future,  we  may  make  their  acquaintance. 

Types.  —  In  the  above  examples  we  have  illustrated  all 
the  principal  fundamental  forms  of  combination.  The 
common  characteristic  of  all  these  forms  is  this,  that  in 
each  all  the  elements  of  lower  valence  are  in  combina- 
tion with  one  element,  this  latter  being,  as  it  were,  the 
typical  element  of  the  molecule.  Now,  all  those  com- 
pounds in  which  a  monad  is  the  typical  element  resemble 
each  other  closely;  those  in  which  a  dyad  is  the  typical 
element  also  resemble  each  other  closely,  and  so  through 
the  list.  Thus,  we  have  the  compounds  divided  into 
classes  according  as  a  monad,  dyad,  triad,  tetrad,  or  pentad 
is  the  typical  element.  In  each  class  we  can  select  some 
one  compound  which  is  the  type  of  the  class.  It  is  the 
type  in  the  sense  that  the  kind  of  combination  found  in 


104  CHEMICAL    COMPOUNDS. 

it  is  found  repeated  in  all  the  members  of  the  class.     We 
may  thus  select  the  five  following  types  :  — 

I.  II.  III.  IV.  V. 

C1 
,H  H 


H—  H,     H—  0—  H,      N—  H  ,        C          ,       P—C1  . 

\H  \>C1 

XC1 

Hydrogen.  Water.  Ammonia.  Methane.  Phosphorus 

chloride. 

Now,  by  imagining  the  constituents  of  these  types 
replaced  by  other  atoms  of  equal  valence,  we  can  obtain 
all  the  examples  above  given,  and  a  very  large  number 
in  addition.  Thus,  H  —  Cl  may  be  looked  upon  as  de- 
rived from  H  —  H  by  the  replacement  of  one  atom  of  II 
by  Cl,  an  atom  of  equal  valence.  So,  also,  Ca=0  may 
be  looked  upon  as  derived  from  H  —  0  —  H  by  the  replace- 
ment of  two  atoms  of  H  by  one  atom  of  Ca,  the  valence 
of  which  is  equal  to  that  of  two  atoms  of  H.  Or,  the 
typical  element  itself  may  be  replaced  by  another  of  equal 

/H 

valence,  as,  for  instance,  in  P  —  H   ,  which  may  be  looked 

\H 

/H 

upon  as  derived  from  the  type  N  —  H    by  the  replacement 

\H 

of  the  triad  N  by  the  triad  P.     It  is  thus  clear  what  rela- 
tion the  types  bear  to  similar  compounds. 

When  the  so-called  u  theory  of  types"  was  proposed,  a 
great  deal  of  importance  was  attached  to  it.  The  true 
secret  of  chemical  combination  was  supposed  to  have 
been  discovered.  Efforts  were  made  to  refer  every  known 
compound  to  some  one  of  the  types  and  thus  to  classify 
the  compounds.  These  efforts  were  undoubtedly  valu- 
able. They  were  the  necessary  precursors  of  our  present 
views  concerning  the  nature  of  chemical  compounds. 
Through  the  theory  of  types  we  arrived  at  our  present 
conception  of  valence.  As  long  as  the  theory  was  in 
vogue,  it  was  simply  necessary  to  refer  anjr  given  com- 


GENERAL    CONSIDERATIONS.  105 

pound  to  some  particular  type.  As  soon  as  it  was  shown 
to  which  type  a  compound  belonged,  investigation  ceased. 
The  internal  arrangement  of  the  atoms  was  not  inquired 
into.  To  conceive  of  the  valence  of  atoms  is  to  take  a 
step  beyond  the  theory  of  types,  to  find  in  the  atoms 
themselves  the  reason  for  the  types.  Now  that  we  have 
taken  this  step,  it  is  unnecessary  to  retain  the  ideas  of 
types  at  all.  It  has  served  its  purpose  and  led  to  another 
idea,  imperfect  to  be  sure,  but  nevertheless  more  perfect 
than  its  predecessor.  At  present,  it  is  not  only  necessary 
to  show  the  resemblance  between  some  molecule  and  a 
typical  molecule,  but,  in  every  case,  we  have  to  determine 
which  elements  are  in  combination  with  each  other. 
When,  according  to  this  latter  principle,  the  constitution 
of  a  compound  is  determined,  resemblances  will  show 
themselves  between  molecules  belonging  to  the  same 
type,  but  these  resemblances  will  only  be  necessary  con- 
sequences of  the  resemblances  between  the  typical  atoms 
of  these  molecules. 

Residues. — By  far  the  greater  number  of  chemical 
compounds  are  more  complicated  than  those  with  which 
we  have  thus  far  been  dealing.  Let  us  inquire  into  the 
cause  of  the  complexity  noticed  in  them. 

If  we  take  any  of  the  formulas  above  given,  as,  for 

/H  H 

/  /TI 

instance,     H— Cl,     H— 0— H,     N— H  ,  and    C  <5   , 

\  \ 

\H  XH 

and  divide  them  at  any  part,  we  obtain  two  residues  of 
equal  valence.  Thus,  if  we  divide  H — Cl,  we  obtain  II 
and  Cl,  both  univalent ;  if  we  divide  H — 0 — H,  we 
obtain  II  and  OH,  and  these  are  both  univalent,  for,  as 
can  be  readily  seen,  the  group  OH  requires  a  univalent 
atom  or  group  to  saturate  it ;  and  this  is  what  we  under- 


stand  by  a  univalent  group.      If  we  divide    N — H  ,    we 

obtain  II  and  NH.2,  or  H2  and  Nil;  by  the  former  division 
there  are  left  two  univalent,  b}T  the  latter  two  bivalent 


106 


CHEMICAL    COMPOUNDS. 


factors.     And  so  in  the  case  of 


H 
/H 

^H 
XH 


if  we  divide 


this  formula,  the  following  cases  are  possible:  H  and 
CH3,  H2  and  CH2,  H3  and  CH;  leaving  in  the  first  case 
two  univalent,  in  the  second  two  bivalent,  and  in  the 
third  two  trivalent,  factors.  This  principle  may  be 
carried  out  further  in  connection  with  other  and  more 
complicated  formulas,  and  thus  are  obtained  the  formulas 
of  a  great  variety  of  these  so-called  residues;  in  most 
cases,  however,  the  division  made  and  the  residues  re- 
sulting may  be  compared  to  the  simpler  forms  described. 
We  speak  of  a  water  residue,  OH,  which,  on  account  of 
the  exceedingly  important  part  it  plays  in  the  constitu- 
tion of  chemical  compounds,  has  received  a  distinct 
name,  hydroxyl;  the  ammonia  residue,  NH2,  is  called 
amide;  the  residue  NH  is  called  imide;  the  methane 
residue,  CH3,  is  called  methyl;  the  residue  CH2,  methy- 
lene,  etc.  etc. 

If  we  operate  with  the  groups  mentioned  instead  of 
with  atoms  alone,  we  shall  find  that  we  are  able  to  build 
up  a  large  number  of  formulas  representing  known  com- 
pounds, as  follows: — 

/H 

Similar  to      N — H  ,    ammonia,  we  have: — 


/"  ^o 

N— H     ,  Nf        , 

\H  X°H 

Hydroxylamine.  Nitrous  acid. 


N— H    ,      p— OH  , 
\H 

Methylamine.        Phosphorous  acid. 


P-CH3  , 


etc. 


Trimethylphosphine. 

H 

/H 
Similar  to    C  <TT  ,     methane,  we  have :— 


GENERAL    CONSIDERATIONS.  10T 


,  X  ^  N 

<  C<  ° 


Methylamine.        Methyl  alcohol.  Formic  acid.  Cyanic  acid. 

//O  CH3 

#'  /TT 

C-OH  ,  C  <g       ,       etc. 

\OH  XH 

?  Ethane. 

These  residues  are  what  were  formerly  known  as 
radicles,  though  the  term  residue,  as  at  present  under- 
stood, is  more  comprehensive  than  the  term  radicle  was. 
The  conception  of  the  residue  is  an  exceedingly  simple 
one,  whereas  there  was  considerable  confusion  in  regard 
to  the  real  meaning  of  the  word  radicle.  It  must  be 
remembered  that  each  of  these  residues  has  a  constitution 
of  its  own,  which  becomes  a  part  of  the  constitution  of 
the  compound  into  which  it  enters  ;  but  if  we  know  the 
constitution  of  the  compound  from  which  the  residue  is 
derived,  we  also  know  the  constitution  of  the  residue. 
These  residues  are  so  well  understood  that,  in  writing 
graphic  formulas,  it  is  customary  to  indicate  their  pre- 
sence by  means  of  formulas  which  are  not  graphic.  For 
instance,  in  the  case  of  the  compound  ethane,  above  given, 


the  complete  graphic  formula  would  be 


C-H 

/  H\ 
C  <      ^H 


but,  having  once  recognized  the  presence  of  the  residue 

/CH, 

CH3,  we  may  write  instead      C  <jj      ;      or,   even   still 

^H 

CH3 

simpler,    |        ,  for,  as  will  be  seen,  the  compound  con- 
CH, 

sists  of  a  combination  of  two  methyl  groups,  CH3. 


108  CHEMICAL    COMPOUNDS. 

Chains. — A  kind  of  combination  which  we  have  not 
yet  considered  consists  in  the  union  of  two  or  more 
atoms  of  the  same  element,  or  of  different  elements,  b}T  a 
part  of  their  affinities,  thus  forming  chains.  Examples 
of  the  first  kind  are  met  with  particularly  in  the  case  of 
carbon.  If  two  quadrivalent  atoms  of  carbon  unite  in 


the  simplest  manner  possible,  we  have  a  group, 


_C— C— , 


which  must  have  six  free  affinities ;  if  three  such  atoms 

i    i    i 

unite,  we  have  a  group,  — C — C — C — ,  which  must  have 

i    i    i 

eight  free  affinities,  etc.;  and,  as  this  chain  combination 
may,  as  far  as  we  know,  be  continued  indefinitely,  and 
the  free  affinities  may  be  saturated  by  the  greatest 
variety  of  atoms  and  residues,  it  is  evident  that  the 
number  of  compounds,  the  possibility  of  whose  existence 
is  thus  indicated,  is  unlimited. 

We  have  examples  of  chains  also  among  oxygen  com- 
pounds, as  we  see  in  the  following  compounds: — 

H— 0— 0— H,    Cl— 0-0— H,    Cl— 0— O— 0— H, 

Hydrogen  peroxide.  Chlorous  acid.  Chloric  acid. 

Cl— O— 0— 0— 0— H,  Br— 0— 0— 0— H,      etc. 

Perchloric  acid.  Bromic  acid. 

The  following  are  instances  of  chains  formed  by  the 
union  of  atoms  of  different  elements: — 

Cl— S— S— Cl,      Cl-S— 0— Cl,      Cl— Se— 0— Cl.  • 

Sulphur  chloride.  Thionyl  chloride  Selenyl  chloride. 

H— 0— S— 0— 0— H,      H— 0— 0-S— 0— 0— H. 

Sulphurous  acid  (?)  Sulphuric  acid. 

All  the  chains  above  given  are  of  the  variety  known  as 
open  chains.  Thus,  in  the  compound  Cl — 0 — O — 0 — H, 
the  chain  is  — 0 — O — 0 — .  At  both  its  ends  we  have 
free  affinities,  capable  of  holding  univalent  atoms.  If 
these  two  free  affinities  were  to  act  upon  each  other,  we 

O 

might  represent  the  resulting  molecule  thus       /   \     . 

O O 


GENERAL    CONSIDERATIONS.  109 

Here  we  have  an  example  of  a  closed  chain.  If  our 
conceptions  in  rega.rd  to  the  nature  of  ozone  are  correct, 
we  have  in  it  the  above  arrangement  of  atoms,  viz., 

O 

\     .     Other  examples  of  closed  chains  are  seen  in 
-O 


the  formulas  — 

O  O  O 

S      I  ,  S/      >0  ,          Ba      |  ,        etc. 


0  0 

Sulphur  dioxide.        Sulphur  trioxide.  Barium  dioxide. 

Double  and  Treble  Union.  —  We  have  alread}^  seen 
that  one  atom  may  be  combined  with  another  by  means 
of  more  than  one  affinity  acting  in  each.  In  the  com- 
pound Ca=O,  for  instance,  two  affinities  of  one  atom  are 
saturated  by  two  affinities  of  another.  This  kind  of  union 
may  occur  in  connection  with  atoms  that  have  a  higher 
valence  than  two.  It  is  particularly  met  with  in  carbon 
compounds,  as  in  C2H4,  ethylene.  In  this  compound,  it 
is  believed  that  the  two  carbon  atoms  are  united  by 

H\      /H 

means  of  two  affinities  each,  thus  :         ;>C—  C<( 

H/  \H 

Further,  in  some  compounds,  a  treble  union  is  found 
between  atoms  of  the  same  kind,  as  in  acetylene,  C,H.2,  in 
which  it  is  believed  that  the  carbon  atoms  are  united  by 
means  of  three  affinities  each,  as  follows:  — 


Lastly,  in  some  compounds,   it   is  believed   that  the 
carbon  atoms  are  united  partially  by  double  and  par- 

CH 


tially  by  single  union,  as,  for  instance,  in    |          ,    which 


is  the  formula  usually  accepted  for  acrylic  acid. 
10 


110  CHEMICAL    COMPOUNDS. 

There  are  also  a  large  number  of  compounds  in  which 
the  carbon  atoms  are  supposed  to  be  united  alternately 
by  double  and  single  union,  and  to  form  a  closed  chain, 
as  in  benzene : — 

H 
C 

/-   \ 
HC         CH 

I1C         CH 

V 

H 


We  have  thus  illustrated  the  manners  in  which  atoms 
are  believed  to  combine  to  form  the  various  chemical 
compounds  with  which  we  have  to  deal.  We  are  far  from 
asserting  that  the  formulas  which  we  have  given  are  in 
all  cases  satisfactorily  proved.  We  entertain  serious 
doubts  in  regard  to  many  of  them.  '  Nevertheless,  bearing 
in  mind  what  is  meant  by  the  constitution  of  compounds, 
we  believe  that  the  formulas  given  represent,  in  the 
majority  of  cases,  truths  which  are  capable  of  proof. 
The  proofs  must,  of  course,  in  each  case,  remain  relative, 
and  cannot  be  absolute.  We  start  witli  certain  assump- 
tions of  atoms  and  the  nature  of  atoms,  and  form  a  con- 
ception in  regard  to  the  constitution  of  the  simplest 
compounds.  Now,  in  so  far  as  this  conception  is  true, 
in  just  so  far,  in  the  majority  of  cases,  we  can  prove  the 
constitution  of  compounds.  Just  as  soon  as  our  ideas 
in  regard  to  the  constitution  of  hydrochloric  acid  change, 
our  ideas  of  the  constitution  of  all  other  compounds 
must  also  change.  Whatever  is  true  in  our  present  con- 
ception of  the  constitution  of  hydrochloric  acid,  is  also 
true  in  our  conception  of  the  constitution  of  other  chem- 
ical compounds. 

The  proofs  of  the  chemical  constitution  of  compounds 
are  of  two  kinds : — 

1.  Those  which  depend  upon  decomposing  the  com- 
pounds into  simpler  constituents,  or  the  analytical  proofs, 
and 


GENERAL    CONSIDERATIONS.  Ill 

2.  Those  which  depend  upon  building  the  compounds 
up  from  simpler  constituents,  or  the  synthetical  proofs. 

In  many  cases  we  are  able  to  obtain  both  of  these 
proofs,  and  if  then  we  reach  the  same  results  b}'  both 
methods,  these  results  are  rendered  doubly  sure.  In 
many  cases,  however,  only  one  kind  of  proof  can  be  given 
at  present,  but  this  ma}*  be  so  strong  that  the  results  are 
satisfactory.  Where  all  proof  is  wanting,  it  is  sometimes 
possible  to  propose  a  formula  which  very  probably  repre- 
sents the  constitution.  Of  this  latter  kind  of  formulas, 
viz.,  those  which  have  not  been  proved,  but  may  be  shown 
to  be  extremely  probable,  we  have  a  great  many. 

The  methods  of  proof  will  be  fully  illustrated  in  the 
following  section,  in  which  we  propose  to  consider  the 
formulas  of  most  of  those  chemical  compounds  which 
represent  classes.  The  proofs  for  the  formulas  commonly 
accepted  will  be  given  in  each  case  as  fully  as  is  com- 
patible with  a  work  of  this  nature.  We  do  not  necessarily 
undertake  to  give  all  the  proofs,  though  we  shall  give 
enough  to  show  clearly  upon  what  foundations  our  con- 
stitutional formulas  are  based — enough  to  show  that 
these  formulas  are  entirely  worthy  of  our  careful  study 
and  our  respect. 


CLASSES  OF  COMPOUNDS. 

Chemical  compounds  may  be  most  conveniently  clas- 
sified according  to  their  chemical  properties.  No  system 
of  classification  which  has  been  proposed,  up  to  the  pre- 
sent, can,  in  any  sense,  be  called  perfect,  and  yet  the  sys- 
tem now  in  most  common  use  is  convenient,  and  has  a 
fair  foundation  in  facts. 

If  we  examine  the  compound  which  has  the  formula 
HC1,  hydrochloric  acid,  and  the  compound  which  has  the 
formula  KOH,  potassium  hydroxide,  we  find  that  the  two 
compounds  differ  very  markedly  from  each  other.  The 
former  has  a  taste  which  we  call  sour,  the  latter  has  the 
taste  of  lye,  or  an  alkaline  taste.  The  former  will  turn 
the  color  of  many  organic  substances,  while  the  latter 
will  undo  the  work  done  by  the  former,  restoring  the 


112  CHEMICAL    COMPOUNDS. 

original  color.  In  whatever  way  we  may  consider  these 
two  compounds,  we  shall  find  that  they  have  opposite  or 
complementary  properties.  They  are  both  chemically 
active  substances,  capable  of  producing  marked  changes 
in  large  numbers  of  other  compounds.  If  they  are 
brought  together  they  neutralize  each  other,  that  is  to 
sa}7,  they  destroy  each  other's  active  properties  and  give 
rise  to  the  formation  of  a  new  compound,  differing  en- 
tirely from  the  two  which  gave  it  birth.  The  two  com- 
pounds, hydrochloric  acid,  HC1,  and  potassium  hydrox- 
ide, KOH,  are  representatives  of  two  great  classes  of 
compounds  known  as  acids  and  banes.  Many  of  the 
members  of  these  two  classes  possess  just  as  marked 
properties  as  do  the  two  which  have  been  mentioned, 
and  for  these  the  subdivision  into  acids  and  bases  is 
rational  and  simple.  But  there  are,  further,  some  com- 
pounds which  appear  to  possess  the  characteristics  of 
both  classes  to  a  certain  extent,  and  of  neither  class  to 
any  very  great  extent.  For  these  compounds  the  system 
is  not  broad  enough,  though  their  number  is  not  great. 
Probabl}7,  when  the  nature  of  the  two  classes  of  com- 
pounds is  fully  understood,  no  difficulty  will  be  found 
in  determining  to  which  class  any  given  compound 
belongs. 

Acids. — The  properties  which   characterize   acids  are 
the  following: — 

1.  They  have  an  acid  or  sour  taste. 

2.  They  change  blue  litmus  red. 

3.  They  act  upon  metals,  hydrogen  being  evolved,  and 
its  place  being  taken  by  the  metals,  as,  for  instance : — 

2(HC1)       -f       Zn         =        ZnCl        +       2H 

Hydrochloric  acid.  Zinc  chloride. 

HaSO4       +       MB       ==       MnSO4       +       2H 

Sulphuric  acid.  Manganese  sulphate. 

4.  They  act  upon  metallic  hydroxides,  forming  neutral 
substances  and  water,  as  follows: — 

HC1       -f       KOH        =        KC1       -[-      H2O 

Hydrochloric  Potassium  Potassium 

acid.  hydroxide.  chloride. 

HNO,        +        NaOH      =     NaNO,        +        H2O 

Nitric  acid.  Sodium  hydroxide.         Sodium  uitrate. 

H,SO4        +       Ca(OH),       =       CaS04       +       2H.20 

Sulphuric  acid.  Calcium  oxide.  Calcium  sulphate. 


GENERAL    CONSIDERATIONS.  113 

Hydrogen  Acids. — All  acids  contain  hydrogen.  They 
may  consist  of  hydrogen  and  only  one  other  element,  or 
of  hydrogen  and  a  group  of  other  elements  of  greater  or 
less  complexity.  The  constitution  of  those  acids  which 
consist  of  hydrogen  and  only  one  other  element  is,  of 
course,  very  simple  and  readily  understood.  There  are 
but  few  examples  of  this  kind,  some  of  which  follow: 
Hydrochloric  acid,  HC1 ;  hydrobromic  acid,  HBr;  sulph- 
ydric  acid,  H^S,  etc.  According  to  our  conceptions  of 
the  nature  of  chemical  constitution,  compounds  of  the 
above  formulas  can  only  have  one  constitution. 

It  is  a  noticeable  fact  that  acids  of  this  first  and  sim- 
plest class  never  contain  more  than  two  atoms  of  hydro- 
gen in  the  molecule ;  or,  that  no  element  with  a  higher 
valence  than  two  forms  these  simple  acids. 

Hydroxyl  Acids. — By  far  the  greater  number  of  acids 
belong  to  the  second  class  mentioned.  They  consist  of 
hydrogen  and  a  group  of  greater  or  less  complexity,  as, 
for  instance,  H(NO3),  nitric  acid  ;  H(C10:t),  chloric  acid; 
H2(SO4),  sulphuric  acid,  etc.  In  nearly  all  acids  of  this 
kind,  oxygen  is  one  of  the  constituents  of  the  group  with 
which  the  hydrogen  is  combined. 

The  hydrogen  in  these  compounds  is  the  changeable 
constituent.  It  is  readily  given  up,  and  metals  and 
groups  are  taken  up  in  its  place.  The  first  question  that 
would  suggest  itself,.in  considering  the  constitution  of 
acids,  would  be  this:  In  what  manner  is  the  hydrogen 
in  them  held  in  combination  :  It  is  believed  that  investi- 
gations thus  far  made,  justify  the  answer  that  the  hydro- 
gen  in  these  acids  is  almost  always  in  combination  with 
oxygen  and,  in  a  very  few  cases,  with  that  element  which 
is  so  similar  to  oxygen,  viz.,  sulphur.  The  proofs  for 
this  statement  cannot  always  be  given.  In  the  cases  of 
many  acids,  there  exist  no  independent  proofs  that  in 
these  the  hydrogen  is  combined  with  oxygen.  On  the 
other  hand,  there  are  so  many  acids  in  which  it  can  be 
satisfactorily  shown  that  the  hydrogen  is  in  combination 
with  oxygen  that  the  above  answer  seems  to  be  justified. 
We  accordingly  write  the  formulas  of  acids  in  such  a 
way  as  to  indicate  the  fact  of  union  between  oxygen  and 
Ii3Tdrogen  thus: — 

10* 


114  CHEMICAL    COMPOUNDS. 

(HO)NO,  (HO)C102  (HO)aSO, 

Nitric  acid.  Chloric  acid.  Sulphuric  acid. 

Or  these  same  formulas  may  be  made  still  more  defi- 
nite by  writing  them  as  follows  : — 

H_0X 

H— 0— NO.,        H— O— C102,  >S02. 

H— <K 

Proofs. — Under  certain  circumstances,  an  atom  of 
oxygen  and  an  atom  of  hydrogen  may  be  removed  from 
an  acid  containing  oxygen,  and  one  atom  of  chlorine 
then  enters  into  the  place  occupied  by  the  displaced 
atoms,  and  is  held  in  combination.  Now,  the  simplest 
conclusion  we  can  draw  is  that  the  oxygen  and  hydrogen 
Were  present  in  the  compound  as  a  univalent  group,  viz., 
as  (OH),  orhydroxyl. 

We  have  the  following  instances  : — 

OH 

yields  the  compounds — 
OH 

1  /Cl 

SO./  and  SO0 


OH  XCI  " 

Sulphnryl  oxichloride.  Sulphuryl  chloride. 

,OH  Cl 

PO—  OH  yields  PO—  Cl  . 


Phosphoric  acid.  Phosphorus  oxichloride. 

C,H30(OH)  yields  C,H3O—  Cl,      etc. 

Acetic  acid  Acetyl  chloride. 

Another  reaction,  which  shows  plainly  that  in  these 
acids  hydrogen  is  intimately  associated  with  oxygen,  is 
that  by  which  the  group  NHa  is  introduced  into  them  in 
the  place  of  an  atom  of  oxygen  and  an  atom  of  hydrogen. 
The  fact  that  the  elements,  oxygen  and  l)37drogen,  are 
displaced  together  indicates  a  connection  between  them 
in  the  compound. 

We  have  the  following  instances  :  — 

C2H30(OH)  yields  C2HaO(NH2) 

Acetic  acid.  Acetamide. 

C7H50(OH)  yields  C7H50(NH2). 

Beuzoic  acid.  Beuzamide. 


GENERAL    CONSIDERATIONS.  115 

These,  with  other  general  reactions,  make  up  the  proof 
of  the  statement  above  made,  that,  in  most  of  those  acids 
which  contain  oxjrgen,  the  characteristic  hydrogen  is  in 
combination  with  oxygen  in  the  form  of  hydroxyl  (OH  •. 
In  some  few  cases,  as  already  mentioned,  the  oxygen  is 
replaced  by  sulphur. 

Further  Experiments  necessary  in  most  Cases.  —  If  we 
accept  the  presence  of  hydroxyl  in  oxygen  acids,  we  are 
prepared  to  take  another  step.  This  hydroxyl  may  be 
in  combination  with  only  one  element  or  with  a  group 
of  elements.  If  it  is  in  combination  with  only  one  ele- 
ment, the  constitution  of  the  resulting  acid  is  easily 
understood.  For  instance,  in  the  compound  C1OH, 
hypochlorous  acid,  only  one  method  of  combination  sug- 
gests itself,  viz.,  Cl  —  0  —  H.  There  are  very  few  examples 
of  this  kind. 

In  those  acids  in  which  the  hydroxyl  is  in  combination 
with  a  group,  the  constitution  is  only  then  determined 
when,  in  addition  to  showing  the  presence  of  hydroxyl, 
the  special  constitution  of  the  group  itself  is  determined. 

In  sulphuric  acid,  for  instance,  after  having  determined 
the  presence  of  two  hydroxyl  groups,  we  have  the  for- 


. 

mula      S02<^  ;      but  this  formula  only  partially  ex- 

H)H 

presses  the  constitution  of  the  acid.  It  remains  to  be 
shown  in  what  manner  the  atoms  are  combined  in  the 
group  SO2,  and  also,  with  what  atoms  the  hydroxyl 
groups  are  combined.  Under  the  assumption  that  both 
sulphur  and  oxygen  are  bivalent  elements,  the  constitu- 
tion of  sulphuric  acid  may  be  expressed  by  two  different 
formulas,  viz.:  — 

0—  0—  H  S—  O—  H 

S<  and  0< 

\0—  0—  H  \0—  0—  H 

Special  experiments  must  decide  which  of  these  for- 
mulas is  the  correct  one. 

Other  Acids.  —  It  has  been  mentioned  that,  in  some 
acids,  sulphur  plays  the  part  which  oxygen  plays  in  the 
hydroxyl-acids.  In  these  we  have  the  uiiivalent  group 


116  CHEMICAL    COMPOUNDS. 

(SH).  The  grounds  for  assuming  the  presence  of  this 
group  in  a  compound  are  similar  to  those  which  lead  to 
the  assumption  that  the  group  (OH)  is  present.  The  two 
atoms  S  and  H  can  both  be  removed  from  the  compound 
and  be  replaced  by  one  univalent  atom,  as  chlorine ;  and, 
further,  there  is  a  general  tendency  for  these  two  ele- 
ments, sulphur  and  hydrogen,  to  leave  the  compound  in 
company.  Examples  of  acids  of  this  kind  are 

ySH 

S02<  and  CN(SH). 

M)H 

Hyposulphurous  acid.  Salphocyanic  acid. 

Lastly,  there  is  one  acid  which  can  be  classified  under 
none  of  the  above  heads.  It  contains  hydrogen  probably 
in  combination  with  carbon,  the  latter  being  at  the  same 
time  in  combination  with  nitrogen.  This  is  hydrocyanic 
acid,  It — C^N,  in  which  the  group  CN  acts  like  an 
element,  making  the  compound  analogous  to  hydrochloric 
acid,  HC1. 

Subdivision  of  Acids. — It  will  be  seen  that  different 
acids  contain  different  numbers  of  hydroxyl-groups  in 
their  molecules.  An  acid  which  contains  only  one  such 
group  in  its  molecule  has,  of  course,  only  one  character- 
istic hydrogen  atom.  It  is  called  a  monobasic  acid.  An 
acid  which  contains  two  such  groups  in  its  molecule  is  a 
bibasic  acid.  We  have,  further,  tribasic,  tetrabasic  acids, 
etc. 

The  same  distinctions  are  possible  among  those  acids 
which  consist  of  hydrogen  combined  only  with  an  ele- 
ment, and  consequently  do  not  contain  hydroxyl ;  but  as 
of  these  latter  acids  we  have  none  which  contain  more 
than  two  atoms  of  hydrogen  in  the  molecule,  so  we  have 
among  them  only  monobasic  and  bibasic  acids. 

Examples: — 

Monobasic  acids.  Bibasic  acids. 

HC1,  hydrochloric  acid.  ,OH 

NO.2(OH),  nitric  acid.  <  S02<^         ,   sulphuric  acid. 

Cl(OH),  hypochlorous  acid.  XOH 

OH 

C20./          i   oxalic  acid. 


GENERAL    CONSIDERATIONS.  117 

Tribasic  acids.  Tetrabafic  acids 

,OH  P.203(OH)4,  pyrophosphoric 

PO  —  OH  ,  phosphoric  acid. 


/0n 

AsO  —  OH,  arsenic  acid. 


Bases.  —  Bases  have  properties  which  are  the  opposite 
of  those  possessed  by  acids.  They  all  contain  oxygen 
and  hydrogen,  and  these  elements  are  combined  as 
hydroxyl,  as  may  be  shown  in  the  same  way  that  it  was 
shown  for  acids.  The  most  striking  characteristic  of 
bases  is  their  power  to  act  upon  acids,  forming  neutral 
substances  and  water,  as  is  seen  in  the  following  re- 
actions :  — 

KOH      -f-      HN03      ==      KNO,      -f      H2O 

Potas-ium  Nitric  Potassium 

hydroxide.  acid.  nitrate. 

Ca(OH)2     +     H2S04     =     CaS04     +     2H2O 

Calcium  Sulphuric  Calcium 

hydroxide.  ac.d.  sulphate. 

Almost  all  bases  consist  of  a  metal  combined  with 
hydroxyl.  Some  few  consist  of  a  group  of  atoms  com- 
bined with  hydroxyl. 

According  to  the  valence  of  the  metals  with  which 
the  hydroxyl  is  combined,  we  have  bases  with  one,  two, 
three,  etc.,  hydroxyl-groups  in  the  molecule.  Examples 
of  these  are  the  following  :  — 

K(OH),  potassium  hydroxide.  A1(OH)3,  aluminium  hydroxide. 

Na(OH),  sodium  hydroxide.  Cr(OH)3,  chromium  hydroxide. 

Ca(OfI)3,  calc'nm  hydroxide.  Ti(OH)4,  titanium  hydroxide 

Ba(OH)3f  barium  hydroxide.  Zr(OH)4,  zirconium  hydroxide. 

Differences  between  Acids  and  Bases.  —  The  difference 
between  acids  and  bases  is  dependent  upon  the  nature  of 
the  elements  or  groups  with  which  the  hydroxyl  is  com- 
bined. The  hydroxyl  compounds  of  those  elements 
which  have  a  markedly  metallic  character  are  bases. 
The  hydroxyl  compounds  of  those  elements  which  have 
a  markedly  non-metallic  character  are  acids.  If  we  con- 
sider the  hydroxyl  compounds  of  those  elements  which 


118  CHEMICAL    COMPOUNDS. 

are  neither  markedly  metallic  nor  non-metallic,  we  find 
that  they  sometimes  act  as  acids  and  sometimes  as  bases. 
Thus  the  compound  SbO(OFT,  antimonyl  hydroxide,  is 
a  weak  base  and  a  weak  acid,  exhibiting  one  property  or 
the  other  according  to  the  nature  of  the  compound  with 
which  it  is  brought  in  contact. 

Complex  Bases.— -As  above  stated,  there  are  a  few 
bases  which  consist  of  hydroxyl  combined  with  a  group 
of  atoms.  Such,  for  instance,  are 

BiO(OH)  UO(OH)  TiO(OH), 

Bismuthyl  hydroxide.      Dranyl  hydroxide.  Titanyl  hydroxide. 

In  these  compounds,  as  in  the  corresponding  acids,  the 
constitution  of  the  group  must  be  determined  before  we 
know  the  constitution  of  the  compound.  In  the  above 
cases,  this  determination  is  a  simple  matter.  Bismuth 
conducts  itself  usually  as  a  trivalent  element;  hence  with 
the  simplest  kind  of  combination  the  group  — Bi=0  will 
be  univalent.  Consequently  the  compound  has  the  for- 
mula 0=Bi— 0— H. 

Salts. — The  neutral  substances  to  which  reference  has 
been  made  as  being  formed  by  the  action  of  acids  upon 
bases  are  called  salts.  Salts  may  be  considered  either  as 
acids  in  which  the  hydrogen  has  been  replaced  by  a  base 
residue,  or  as  bases  in  which  the  hydrogen  has  been  re- 
placed by  an  acid  residue.  As  the  base  residues  are 
usually  simpler  than  the  acid  residues,  the  former  view 
is  most  commonly  held,  although  the  two  views  are,  of 
course,  identical. 

It  is  a  simple  matter  to  deduce  the  constitution  of  a 
salt  from  that  of  the  acid  and  base  or  bases  from  which 
it  is  derived.  Usually  the  hydrogen  of  the  acid  is  re- 
placed by  one  or  more  metals,  the  latter  being  held  in 
combination  by  the  same  force  or  forces  that  held  the 
former.  Thus  we  have  * 

/OH  /OK 

S02<  and  S02< 

M)H  H)K 

Sulphuric  acid.  Potassium  sulphate. 

NO,— OH  and  NO,— ONa 

Nitric  acid.  Sodium  nitrate. 


GENERAL    CONSIDERATIONS.  119 

Or,  a   bivalent   element   may   enter  into   an  acid,  in 
which  case  two  hydrogen  atoms,  will  be  replaced,  thus  : — 

/OH  /O 

SO./  and  SO/      >Ca    ; 

\OH  ^O/ 

Sulphuric  acid.  Calcium  sulpbate. 


2— OH  and  >Ba  . 

NO— O/ 

Nitric  acid.  Barium  nitrate. 

Further  complications  are  introduced  when  trivalent 
and  quadrivalent  elements  enter  into  the  composition  of 
salts.  From  what  has  been  said,  however,  the  consti- 
tution of  these  salts  will  be  readily  understood. 

Complex  Salts. — Just  as  we  have  a  few  bases  which 
consist  of  hydroxyl  combined  with  groups  of  atoms,  so 
we  have  salts  which  may  be  considered  as  derived  from 
acids  by  the  replacement  of  hydrogen  by  groups  of  atoms. 
Thus,  a  salt  obtained  from  the  acid  NO, — OH,  and  the 
base  UO— OH,  has  the  constitution  expressed  by  the 
formula  N02 — O — UO.  Here  the  hydrogen  of  the  acid 
is  replaced  by  the  group  UO,  which  is  univalent. 

Anhydrides. — The  constituents  of  water  may  be  ab- 
stracted from  many  acids,  and  thus  is  formed  a  new  class 
of  compounds  called  anhydrides.  The  most  striking- 
characteristic  of  these  compounds  is  their  power  to  form 
acids  with  water,  or  to  form  salts  by  direct  union  with 
bases.  The  following  are  examples  of  anhydrides  :  Sul- 
phuric anhydride,  SO3;  nitric  anhydride,  N205;  phos- 
phoric anhydride,  P205 ;  acetic  anhydride  (C2H}0)2O,  etc. 

When  an  anhydride  is  formed  from  a  monobasic  acid, 
two  molecules  must  combine  to  furnish  the  hydrogen  for 
the  water.  After  the  abstraction  of  the  water,  the  two 
acid  residues  remain  united,  through  the  instrumentality 
of  an  atom  of  oxygen,  thus : — 

NO.-OH)  N02X 

I       -      H20       =  )0    ; 

N02— OH)  NO/ 

2  molecules  Nitric  acid.  Nitric  anbydride. 


120  CHEMICAL    COMPOUNDS. 

C3H30— OH)  C.,H3CK 

H,0     =  >0  . 

C.2H30— OH  )  0,H3OX 

2  molecules  Acetic  acid.  Acetic  anhydride. 

When  an  anhydride  is  formed  from  a  bibasic  acid,  a 
molecule  of  water  may  be  given  off'  from  a  molecule  of 
acid,  thus: — 

on 

SO/         .  —     H^O  S0=0   ; 

M)H 

Sulphuric  acid.  Sulphuric  anhydride. 

/OH 

C0<  —     HO  C0=0  . 

X)H 

Carbonic  acid  (hypothetical).  Carbonic  anhydride. 

Or,  two  molecules  of  a  bibasic  acid  may  unite  and  give 
off  one  molecule  of  water,  forming  a  compound  which  is, 
at  the  same  time,  an  acid  and  an  anhydride,  thus : — 

/OH  ]  /OH 

so/  so/ 

-      H.O      =  >0     .   .    . 

S°OH|  .     '         S°'\OH 

2  molecules  Sulphuric  acid.  Pyrosulphuric  acid. 

When  an  anhydride  is  formed  from  a  tribasic  acid, 
several  possibilities  may  present  themselves.  1.  One 
molecule  of  the  acid  may  lose  one  molecule  of  water,  a 
compound  being  formed  which  is  anhydride  and  acid, 
thus: — 

OH  ^O 

PO— OH      —      H2o      =      ro— OH 

\ 

^OH  Metapho*phoric  acid. 

Phosphoric  acid. 

42.  Two  molecules  of  the  'acid  may  lose  one  molecule 
of  water,  a  compound  being  formed  which  is  a  tetrabasic 
acid,  and,  at  the  same  time,  an  anhydride: — 


GENERAL    CONSIDERATIONS. 


121 


,OH    1  X)H 

PO— OH  PO— OH 

N> 

>    -  H'°        -          PO/OH 

\OH 

Pyrophosphoric 

acid. 
2  mol.  Phosphoric  acid. 

3.  Two  molecules  of  the  acid  may  lose  three  molecules 
of  water,  a  complete  anhydride  being  formed: — 


J&SL 

PO— OH 


PO— OH 
\3H 

PO— OH 


,0 


—         3H.O         == 


2  mol.  Phosphoric  acid. 


PO 
PO 


Phosphoric 
anhydride. 


By  combining  a  larger  number  of  molecules  of  the 
acids  and  abstracting  different  numbers  of  molecules  of 
water,  a  great  variety  of  anhydrides  might  be  produced, 
at  least  theoretically.  Not  many  such  complicated  pro- 
ducts are  positively  known,  however. 

From  tetrabasic  acids  and  acids  with  even  higher 
basicity,  corresponding  anhydrides  maybe  derived.  With 
an  increase  in  the  basicity  of  the  acids,  the  complexity 
of  the  resulting  anhydride  is,  of  course,  increased. 

Proofs  of  the  Constitution  of  Anhydrides. — In  regard 
to  the  correctness  of  the  formulas  given  for  these  anhy- 
drides, it  can  only  be  said  that  they  are  the  simplest 
which  we  can  conceive  of.  If  we  acknowledge  that 
acetic  anhydride,  (C2H3O)yO,  consists  of  two  acid-residues 
combined  by  an  ox_ygen  atom,  then,  by  analogy,  we  must 
acknowledge  that  the  other  anhydrides,  mentioned  above, 
are  constituted  as  represented  by  the  formulas.  But, 
can  we  assume  any  other  formula  for  acetic  anhydride  ? 
We  know  that  the  acid  has  the  constitution  C2H3O(OH); 
11 


122  CHEMICAL    COMPOUNDS. 

we  know  that  the  anhydride  has  the  empirical  formula, 
C4HHOH,  and  that  it  is  formed  by  the  simple  abstraction 
of  water  from  the  acid  ;  we  know  that  the  hydroxyl 
group  has  the  power  to  separate  from  the  acid  with  compa- 
rative ease.  What,  then,  is  more  natural  than  to  assume 
that  the  water  which  is  given  off  from  the  acid  is  formed 
from  the  hydroxyl  groups,  and  that  the  groups  C2HH0 
remain  undecomposed  ?  But  this  would  give  us,  besides 
the  water,  two  groups,  C.2H3O  and  an  oxygen  atom. 
These  are  all  combined  in  one  molecule,  and,  as  we 
believe,  in  such  a  way  that  the  oxygen  atom  exercises  a 
linking  power  between  the  two  groups  or  acid  residues, 

C.fl.,0 

giving  the  formula  —  )O  . 

C2H3(K 

When  an  anhydride  is  formed  by  the  abstraction  of 
water  from  one  molecule  of  an  acid,  the  simplest  conclu- 
sion we  can  draw  is  that  an  oxygen  atom  fills  the  place 
before  occupied  by  two  hydroxyl  groups.  We  have  no 
proof  of  this,  to  be  sure,  but  it  would  be  gratuitous  to 
offer  any  other  explanation  of  the  formation  of  these 
anhydrides  at  present. 

Oxides.  —  Just  as  anhydrides  may  be  obtained  from 
acids  by  the  abstraction  of  water,  so  the  oxides  may  be 
regarded  as  anh}Tdrides  of  the  bases.  The  consideration 
of  the  oxides  is  simpler  than  that  of  the  anhydrides, 
because  the  bases  themselves  are  generally  simpler  than 
the  acids. 

The  simplest  oxides  are  .those  obtained  from  the 
hydroxides  of  univalent  elements,  examples  of  which 
follow  :  — 

KOH)  Kx 

-         H,0         =  \0 

KOH)  tf 

2  molecules  Potassium  Potassium 

hydroxide.  oxide. 


)  Nax 

H20  >0  . 

Na—  OH  )  Na/ 

Sodium  hydroxide.  Sodium  oxide. 


GENERAL    CONSIDERATIONS.  123 

Of  oxides  obtained  from  the  hydroxides  of  bivalent 
elements,  we  have,  among  others,  the  following:  — 

/OH 

Ca(  —         H20         =        C=0  ; 


Calcium  hydroxide.  Calcium  oxide. 

/OH 

Sr<  —         H.O         =         Sr=0  . 

M)H 

Strontium  hydroxide  Strontium  oxide. 

Theoretically,  an  intermediate  anhydride  mo3r  be  de- 
rived, from  either  of  the  two  proceeding  oxides,  analogous 

/OH 

so/ 

to  the  formation  of  pyrosulphuric  acid,  j>O    ,  from 

so 


two  molecules  of  sulphuric  acid,  thus  :  — 


c/-[    -    B<°      -       > 

-•OK\ 

2  mol.  Calcium  hydroxide.  Intermediate  anhydride. 

No  such  compounds  have  been  obtained  as  yet.  From 
the  hydroxides  of  trivalent  elements  we  may  have  more 
than  one  oxide  formed.  If  one  molecule  of  the  hydroxide 
loses  one  molecule  of  water,  a  body  is  formed  which  is 
oxide  and  l^droxide  at  the  same  time.  Reference  has 
been  made  to  these  compounds  (see  ante,  p.  118),  under 
the  head  of  bases.  The  compound  A10 — OH  may  be 
regarded  as  derived  from  the  hydroxide  A1(OH)3,  by 
the  loss  of  one  molecule  of  water  from  one  molecule  of 
the  hydroxide.  It  is  both  oxide  and  hydroxide. 

The  most  common  method  of  formation  of  oxides  from 
hydroxides  of  trivalent  elements  consists  in  the  union  of 
two  molecules  of  the  hydroxide  to  lose  three  molecules 
of  water,  thus  : — 


124  CHEMICAL    COMPOUNDS. 


Al— OH  Q 


OH 
-OH 


Air 

—     3H20       =  >0 

AU 


^*       "•"•  Aluminium 

\OH 

2  mol.  Alum  nium  hydroxide. 

The  principle  of  the  formation  of  these  oxides  is  thus 
seen  to  be  the  same  as  that  of  the  formation  of  anhy- 
drides. What  was  said  in  regard  to  the  constitution  of 
the  latter  holds  good  in  regard  to  .the  constitution  of  the 
former.  The  view  stated  is  the  simplest  which  the  facts 
permit. 

Analogy  between  Salts  and  Anhydrides  and  Oxides. — 
As  we  saw  above,  a  salt  is  either  an  acid  in  which  the 
hydrogen  is  replaced  by  a  base  residue,  or  a  base  in 
which  the  hydrogen  is  replaced  by  an  acid  residue.  In 
those  salts  which  are  derived  from  acids  containing 
hydroxyl,  we  have  a  base  residue  and  an  acid  residue 
united  by  means  of  oxygen.  On  the  other  hand,  in 
many  anhydrides,  we  have  an  acid  residue  and  an  acid 
residue  united  by  means  of  oxygen,  while  in  oxides,  we 
have  two  base  residues  united  by  means  of  ox}Tgen. 


COMPOUNDS  OF  CARBON. 

We  have  thus  briefly  considered  the  different  classes 
of  compounds,  and  have  shown  upon  what  foundations 
our  ideas  in  regard  to  the  general  constitution  of  these 
classes  of  compounds  rest.  Among  the  compounds  of 
carbon,  we  have  many  representatives  of  each  of  the 
classes  above  considered,  and  all  that  has  been  said  holds 
good  for  these  compounds;  but  owing  to  some  peculiari- 
ties of  carbon,  which  distinguish  it  from  the  other  elements, 
certain  things  hold  good  for  the  carbon  compounds  in 
general,  which  do  not  hold  good  for  the  corresponding 


GENERAL    CONSIDERATIONS.  125 

compounds  of  other  elements.  In  the  following  para- 
graphs, therefore,  the  general  formulas  of  the  different 
classes  of  carbon  compounds  will  be  briefly  treated. 

Hydrocarbons. — Of  the  compounds  of  carbon  those 
which  it  forms  with  h}Tdrogen  are,  in  general,  the  simplest, 
and,  of  the  hydrocarbons,  marsh-gas,  or  methane,  CH4, 
is  the  simplest  one.  With  oar  present  ideas  in  regard 
to  constitution,  there  can  be  but  one  formula  for  this 

H 
compound,  viz.:    H — C — H  ,    which    represents  merely 

H 

that  a  quadrivalent  atom  of  carbon  is  saturated  by  means 
of  four  hydrogen  atoms.  This  is  the  most  rational  sup- 
position which  we  can  make  with  reference  to  this  com- 
pound. The  formula  is  certainly  not  proved,  but  it  is 
exceedingly  probable.  As  marsh-gas  is  a  very  important 
member  of  the  group  of  carbon  compounds,  let  us  inquire 
more  particularly  concerning  the  grounds  upon  which  the 
above  formula  is  based.  We  first  determine  the  empirical 
.formula,  CH4,  by  means  of  analysis,  and  the  determination 
of  the  specific  gravity  of  the  vapor  of  the  compound. 
This  formula  is  the  expression  of  a  fact  and  an  hypothesis. 
The  fact  expressed  is  that  methane  contains  75  per  cent, 
carbon  and  25  per  cent,  hydrogen.  The  hypothesis  is 
that  the  molecules  of  all  chemical  compounds,  in  the 
form  of  gas  or  vapor,  have  thev  same  volume  as  a  mole- 
cule of  hydrogen.  This  hypothesis  tells  us  the  weights 
of  the  atoms  contained  in  the  molecule  of  methane  and 
the  weight  of  the  molecule  of  methane,  and  hence,  further, 
the  number  of  atoms  of  carbon  and  hydrogen  contained 
in  the  molecule.  Knowing  the  above,  it  remains  for  us 
to  determine  in  what  manner  these  atoms  are  united,  or, 
what  is  the  same  thing,  to  determine  which  atoms  are  in 
combination  with  each  other.  From  our  knowledge  of 
hydrogen,  we  assume  that  it  acts  in  this  compound,  as  in 
all  its  other  compounds,  as  a  univalent  element.  But  if 
it  does  act  thus,  then  two  hydrogen  atoms  cannot  be 
united  in  the  molecule  CH4,  for,  if  they  were,  they  could 
not  remain  a  part  of  the  molecule,  as  their  affinities  would 

11* 


126  CHEMICAL    COMPOUNDS. 

be    satisfied    by  their   action    upon  each    other.     Conse- 
quentty,  the  hydrogen  atoms  must  all  be  in  combination 
with  the  carbon  atom.     This  result  we  express  by  the 
H 


[— C— H  , 


formula      H — C — H  ,       which  is,  further,  in  accordance 

H 

with  what  we  know  concerning  carbon,  this  element  being 
in  almost  all  cases  quadrivalent. 

A  question  which  would  naturally  suggest  itself  in 
connection  with  the  compound  CH4  is  this :  Do  all  the 
hydrogen  atoms  play  the  same  part  in  the  molecule  ?  In 
regard  to  this  point,  we  can  only  say  that,  as  far  as 
investigations  go,  an  affirmative  answer  to  this  question 
seems  to  be  justified.  If  these  hydrogen  atoms  were  dif- 
ferent, then,  by  replacing  different  ones  by  the  same  ele- 
ment or  group,  products  should  be  obtained  which  are 
not  identical.  No  such  results  have  been  reached,  although 
the  hydrogen  atoms  in  methane  have  been  replaced  in  a 
great  variety  of  ways. 

Homologous  Series. — Starting  with  methane,  we  have  a 
series  of  hydrocarbons  of  the  general  formula  CnH2«-f2- 
These  resemble  each  other  in  many  respects,  and  differ 
from  each  other  in  their  formulas  in  a  very  simple  way. 
The  difference  in  the  formulas  of  any  two  contiguous 
members  of  this  series  is  CH2.  Such  a  series  is  called 
an  homologous  series.  A  number  of  similar  series  is 
known.  In  the  methane  series  we  have:  Methane,  CH4; 
ethane,  C.,H6 ;  propane,  C3H8 ;  butane,  C4H10,  etc. 

The  general  principle  according  to  which  the  members 
of  this  series  are  formed  is  found  in  the  combination  of 
the  carbon  atoms  in  open  chains.  Thus,  as  we  have  seen 
above,  if  two  carbon  atoms  combine  in  the  simplest 
manner  possible,  viz.,  by  one  of  their  affinities  each,  a 
chain  is  formed  having  six  free  affinities,  as  follows : 

— C — C — .      If  three  carbon  atoms  combine  in  the  same 

i    i 

way,  a  chain  is  formed  having  eight  free  affinities,  thus: 
— C — C — C — .     In   the   same   way,  four   carbon   atoms 

I     !    I 


GENERAL    CONSIDERATIONS.  127 

combining  would  give  a  chain  having  ten  free  affinities, 
etc.  etc.  By  saturating  these  free  affinities  by  hydrogen, 
we  would  get  compounds  of  the  formulas  C.2H6,  CSH8, 
C4H10,  etc.  etc.,  which  are  the  formulas  of  the  hydro- 
carbons above  given. 

Experimental  Proofs. — Certain  experiments  have  been 
performed  which  prove  the  correctness  of  the  views  in 
regard  to  the  nature  of  the  combination  in  the  methane 
series  of  hydrocarbons. 

If  methane  is  treated  with  chlorine,  the  following 
reaction  takes  place : — 

H  H 

H— C— H      4-     Cl— Cl     =     H— C— Cl    +   Cl— H  . 

!  I 

H  H 

Methane.  CLlormethane. 

If  the  product  is  treated  with  sodium,  the  chlorine  is 
extracted,  and  a  compound  of  the  formula  CaH6  is  ob- 
tained, according  to  the  following  equation: — 

H  H  H     H 

H-C— :ci+ Cl:—  C— H  -f   2Na  =  H— C— C— H  +  2NaCl. 

MM  II 

H  H  H     H 

2  molecules  Chlormethane.  Ethane 

With  ethane  similar  reactions  may  be  realized,  and  a 
product,  C3H8,  obtained,  thus  : — 

H  H  H  H 

i    I  II 

1.    H-C-C— H      +      C1-C1        =        H— C-C-C1      +      HC1    ; 

U  il  A 

Ethane.  Chlurethane. 

H   H  H  H    H     H 

2.  H— C-C-  ici  +  Cli— C-H      +    2Na    =     H— C-C-C— H    -f    2NaCl  . 

I     I     i  I  I      '      I 

H    H  H  H    H     H 

Chlorethane.      Chlormethane.  Propane. 

It  is  perfectly  plain  that,  by  continuing  these  reactions 
with  the  new  compounds  obtained,  we  would  have  it  in 
our  power  to  build  up  a  whole  series  of  hydrocarbons 
corresponding  to  the  series  given  above.  If  the  combi- 


128  CHEMICAL    COMPOUNDS. 

nation  always  took  place  in  the  manner  described,  we 
should  have  simple  chains,  in  which  all  the  carbon  atoms 
except  those  at  the  ends,  would  have  two  free  affinities, 
while  those  at  the  ends  would  have  three  free  affinities. 
The  hydrocarbons  themselves  would  be  respectively: — 

H^.CH^.CH^.CH^  H3C.CH2.CH2.CH..CH3,  etc.  etc. 

These  are  called  normal  hydrocarbons,  to  distinguish 
them  from  others  of  the  same  general  composition,  but 
of  different  constitution,  which  will  be  treated  of  in  a 
later  paragraph. 

Alcohols. — Running  parallel  to  the  series  of  hydro- 
carbons which  we  have  just  considered,  is  a  series  of 
compounds  which  maybe  looked  upon  as  derived  from 
the  hydrocarbons  by  replacing  a  hydrogen  atom  in  each 
by  the  univalent  group  OH,  or  hydroxyl.  These  com- 
pounds are  to  organic  chemistry  what  the  hydroxides  of 
the  metals  are  to  inorganic  chemistry.  They  are  known 
as  alcohols.  The  simplest  of  these  is  derived  from 

H 

methane   and   has  the  formula     H — C — 0 — H  . 

A 

Proofs. — One  of  the  hydrogen  atoms  of  those  alcohols 
which  contain  but  one  oxygen  atom,  differs  from  the 
others.  It  is  easily  replaceable  by  certain  groups  known 
as  acid  groups,  which  we  shall  consider  hereafter.  It  is 
also  replaceable  by  metals.  In  a  compound  of  the  for- 
mula CH4O,  we  must  assume  that  one  hydrogen  atom  is 
in  combination  with  the  oxygen  atom,  while  the  other 
three  are  not,  in  orfrer  to  account  for  its  characteristic 
behavior.  Again,  if  we  treat  the  alcohol  with  HC1,  the 
oxygen  atom  and  the  peculiar  hydrogen  atom  are  given 
off  together,  and  their  place  is  taken  by  a  single  atom 
of  chlorine.  This  shows  that  the  hydrogen  and  oxygen 
were  present  in  the  form  of  a  univalent  group,  or  as 
hydroxyl,  which  is  the  on'ly  form  that  satisfies  these 
conditions. 

H,C— OH    -f    H(NO,()      ==      HaC—0(N03)    +    H2O. 

Mtthyl  alcoiiol.  Nitric  acid.  Nitric  e.her.  Water. 


GENERAL    CONSIDERATIONS.  129 

H3C—OH     +      HC1        =        H3C— 01     +     H,0. 

Metbyl  alcohol.  Hydrochloric  Chlorine  thane.  Water. 

acid. 

HSC— OH       +     Na      =      H:iC— ONa     +      H. 

Methyl  alcohol.  Sodium.  Sodium  methylate.  Hydrogen. 

Further,  the  hydroxyl  group  can  be  introduced  into 
the  hydrocarbons  and  the  alcohols  thus  obtained.  In 
order  to  obtain  the  alcohol  CH40,  we  start  from  chlor- 
methane,  CH8C1.  If  this  is  treated  with  the  hydroxide 
of  silver,  the  following  reaction  is  realized: — 

CH3C1      +     Ag(OH)      =      CH:i.OH      +'    AgCl. 

Chlormetbane.  Methyl  alcohol. 

The  above  proofs  suffice  to  show  the  correctness  of  the 

H 

I 
formula     H— C— 0— H     for  the   first  member  of  the 

H 

series  of  alcohols.  Having  once  recognized  the  presence 
of  hydroxyl  in  this  alcohol,  we  would  naturally  expect  to 
find  it  in  the  other  alcohols.  It  is  found  in  them  all,  and 
may  be  detected  in  the  manner  indicated  in  the  case  just 
considered. 

Classes  of  Alcohols. — It  has  been  found  that  there  are 
three  distinct  classes  of  alcohols,  which  have  been  called, 
respectively,  primary,  secondary,  and  tertiary.  These 
differ  from  each  other,  in  their  properties,  very  markedly. 
This  difference  is  particularly  noticed  when  they  are  sub- 
jected to  the  influence  of  oxidizing  agents,  when  they 
undergo  change,  as  follows  : — 

Primary  alcohols  are  converted  first  into  aldehydes, 
and  then  into  acids  containing  the  saVne  number  of  carbon 
atoms. 

Secondary  alcohols  are  converted  into  acetones,  which, 
when  further  oxidized,  yield  acids  with  a  smaller  number 
of  carbon  atoms. 

Tertiary  alcohols  are  decomposed  without  previous 
formation  of  aldehydes  or  acetones,  yielding  acids  with  a 
smaller  number  of  carbon  atoms. 

Primary  Alcohols. — These  differences  in  the  properties 
are  undoubtedly  due  to  differences  in  constitution.  In 


130  CHEMICAL    COMPOUNDS. 

all  primary  alcohols  we  find  that  the  group  CH  .OH,  or 
H 

— C — O — H   ,     is  present.  This  we  saw  in  methyl  alcohol, 

H 

which  is  a  compound  of  this  group  with  hydrogen,  thus: 

H— C— 0— H  .      In 


i 


ethyl  alcohol,  the  next  member  of 


the  series,  this  group  is  also  present.     This  follows  as 
soon  as  we  acknowledge  the  presence  of  hydroxyl  in  the 

H    H 

I       | 
alcohols  ;  for,  in  the  compound       H — C — C — H  ,        it 


i-i 


makes  no  difference  which  hydrogen  atom  is  replaced  by 
hydroxyl,  the  resulting  compound  will,  in  every  case, 
have  the  same  constitution  and  will  necessarily  contain 
the  group  CH...OH.  In  all  alcohols  which  conduct  them- 
selves as  primary,  the  presence  of  the  group  CH  .OH 
can  be  proved  in  a  similar  way.  They  are  all  derived 
from  methyl  alcohol  by  the  replacement  of  a  hydrogen 
atom  with  hydrocarbon  residues  of  various  composition 
and  constitution. 

By  replacing  a  hydrogen  atom  with  methyl,  CH3,  ethyl 
alcohol,  CH  .CH..OH,  is  obtained. 

By  replacing  a  hydrogen  atom  with  ethyl,  C2H.,  propyl 
alcohol,  C,H5.CH.2.OH,  is  obtained. 

By  replacing  a  hydrogen  atom  with  propyl,  C3H8, 
butyl  alcohol,  OtEEg.GHrOH9  is  obtained,  etc. 

Secondary  Alcohols.  —  If  we  replace  two  hydrogen 
atoms  of  methyl  alcohol  with  hydrocarbon  residues, 
alcohols  are  obtained  which  do  not  contain  the  group 
CH^.OH,  as  is  evident  from  the  following  examples: — 


GENERAL    CONSIDERATIONS.  131 

H  CH3  C2H5 

I  I  I 

H— C— 0— H,      H3C— C— 0— H;     H3C— C— O— H  . 

H  H  H 

Methyl  alcohol.  Isopropyl  alcohol.  Secondary  butyl  alcohol. 

These  bodies  contain  the  group  CH.OH,  and  are  repre- 
sentatives of  secondary  alcohols. 

The  simplest  example  of  this  class  of  bodies  is  iso- 
propyl  alcohol,  the  formula  of  which  is  given  above. 

Proofs  of  the  General  Formula  of  Secondary  Alcohols. 
— There  are  two  alcohols  of  the  formula,  C:,HS6.  One  of 
these  conducts  itself  like  the  primary  alcohols,  and  is 
hence  supposed  to  contain  the  group  CH^.OH.  An 
alcohol  isomeric  with  the  primary  alcohol  cannot  contain 
the  group  CH2.OH,  but  must  contain  the  group  CH.OH, 
as  may  be  readily  shown.  Both  of  the  alcohols  are 

H     H     H 

I       I       I 
derived  from  the  same  hydrocarbon,  II — C — C — C — H  . 

H     H    H 

In  this  hydrocarbon  there  are  only  two  kinds  of  hydrogen 
atoms,  viz.,  those  in  combination  with  the  central  carbon 
atom,  and  those  in  combination  with  the  terminal  carbon 
atoms.  If  we  replace  any  one  of  the  latter  by  hydroxyl, 
we  obtain  primaiy  propyl  alcohol  containing  the  group 
CH.7.OH.  Whereas,  if  we  replace  one  of  the  former 
hydrogen  atoms  by  hydroxyl.  we  obtain  secondary 
propyl  alcohol  containing  the  group  CH.OH.  Only 
these  two  cases  are  possible. 

But,  again,  this  secondary  alcohol  is  prepared  by 
allowing  nascent  hydrogen  to  act  upon  acetone.  It  will 
be  shown  that  acetone  can  only  have  the  constitution 

OB. 

C— O  .        Now,    in    being    converted    into    secondary 

CH3 

propyl  alcohol,  acetone  takes  up  two  atoms  of  hydrogen, 
and  the  only  place  where  these  hydrogen  atoms  can  find 


132  CHEMICAL    COMPOUNDS. 

entrance  into  the  above  molecule  is  at  the  central  carbon 
atom.  One  of  the  bonds  of  union  between  the  oxygen 
and  carbon  is  loosened,  and  hydroxyl  is  formed;  thus  the 
carbon  atom  becomes  possessed  of  a  free  affinity,  which 
is  at  once  saturated  with  hydrogen,  and  we  have  the 


:— c— o— 


group  CH.OH  or  H— C— 0— H,  which  is  bivalent. 

Similar  considerations  in  connection  with  other  sec- 
ondary alcohols  lead  to  similar  results,  and  hence  the 
conclusion  is  drawn  that  all  secondary  alcohols  contain 
the  group  CH.OH. 

Tertiary  Alcohols.  —  If  we  replace  three  hydrogen 
atoms  of  methyl  alcohol  by  hydrocarbon  residues, 
alcohols  are  obtained  which  contain  neither  the  group 
CH2.OH,  nor  the  group  CH.OH,  as  may  be  seen  in  the 
following  examples: — 

H  CH3  C.H5 

H— C— 0— H  ,     CH3— C— 0— H  ,     CH3— C— 0— H  . 

I  I  I 

H  CH,  CH3 

Methyl  alcohol.  Tertiary  butyl  alcohol.  Tertiary  amyl  alcohol. 

These  bodies  contain  the  group  C.OH,  and  are  repre- 
sentatives of  tertiary  alcohols. 

The  simplest  example  of  this  class  of  alcohols  is  ter- 
tiary butyl  alcohol,  C4H10O,  the  constitution  of  which  is 
indicated  by  the  formula  given  above. 

Proofs — The  proofs  for  the  correctness  of  this  formula 
are  the  following: — 

There  is  a  hydrocarbon,  the  formula  of  which  can  be 

OH. 

shown  to  be     CH3 — C — CH3  .       From  this,  two  alco- 

H 

hols  are  derived,  one  of  which  conducts  itself  as  a  primary 


GENERAL    CONSIDERATIONS.  133 

alcohol,  and  the  other  of  which  does  not.     The  former 

CH3 

must  have  the  formula     CH3— C— CH2— OH  .          The 


J. 


only  alcohol  derived  from  this  hydrocarbon  which  is  not 
a  primary  alcohol  must  have  the  formula 
CH3 

CH3 — C — CH3  ,     and  hence  contains  the  group 

O 

I 
H 

— 0— OH  ,     which  is  trivalent      Further,  similar  con- 

i 

siderations  of  other  tertiary  alcohols  indicate  that  in 
them  also  the  group  C.OH  is  contained;  and  consequently 
this  is  looked  upon  as  the  characteristic  group  of  these 
bodies. 

Determination  of  Alcohols. — With  the  knowledge  thus 
gained  with  reference  to  alcohols  in  general,  it  is  plain 
that  we  have  it  in  our  power  to  determine  in  any  par- 
ticular case,  1,  whether  the  body  we  are  dealing  with  is 
an  alcohol;  and,  2,  whether  it  is  a  primary,  secondary, 
or  tertiary  alcohol.  The  first  thing  to  be  done  is  to 
determine  whether  the  body  contains  hydroxyl  or  not. 
Treatment  with  the  chlorides  of  phosphorus,  either  the 
terchloride  or  pentachlonde,  is  one  of  the  best  means  of 
making  this  determination.  If  by  treatment  with  the 
chloride  a  product  is  obtained  containing  one  atom  of 
chlorine  in  the  molecule  in  the  place  of  an  oxygen  atom 
and  a  hydrogen  atom,  we  can  assume  that  hydroxyl  was 
present.  If  this  hydroxyl  is  the  alcoholic  hydroxyl,  then 
its  hydrogen  must  be  capable  of  replacement  by  the  so- 
called  acid  groups.  To  determine  this,  acetyl  chloride, 
CSHSO.C1,  is  very  frequently  employed.  If  this  body  is 
allowed  to  act  upon  a  substance  containing  an  alcoholic 
hydroxyl  group,  the  chlorine  of  the  chloride  combines 
12 


134  CHEMICAL    COMPOUNDS. 

with  the  hydrogen  of  the  hydroxyl,  forming  hydrochloric 
acid,  and  the  acid  group  C2HaC)  takes  the  place  of  the 
hydrogen,  thus:  — 

R_OH     -f     C2H3O.C1     =     R_O— G\H,0     -f    HCL 

Alcohol.  Acetyl  chloride.  New  product. 

If  the  reaction  takes  place  in  this  manner,  we  are 
justified  in  concluding  that  the  hydroxyl  group  is  alco- 
holic, or  that  the  body  under  examination  is  an  alcohol. 

It  remains  still  to  determine  whether  the  alcohol  is 
primary,  secondary,  or  tertiarj7.  This  can  be  accom- 
plished by  subjecting  it  to  the  influence  of  oxidizing 
agents. 

If  it  yields  an  aldehyde,  and  then  an  acid  containing 
the  same  number  of  carbon  atoms,  it  is  a  primary  alcohol. 

If  it  yields  first  an  acetone,  and  then  by  further  oxida- 
tion breaks  up,  yielding  an  acid  or  acids  containing  a 
smaller  number  of  carbon  atoms,  it  is  a  secondary  alcohol. 

If.  without  first  yielding  an  aldehyde  or  an  acetone,  it 
breaks  up  directly  with  the  formation  of  an  acid  contain- 
ing a  smaller  number  of  carbon  atoms,  it  is  a  tertiary 
alcohol. 

The  above  tests  then  enable  us  to  determine  a  part  of 
the  constitutional  formulas  of  many  compounds.  If  we 
have  by  means  of  these  tests  determined  that  a  compound 
is  a  primary  alcohol,  we  assume  that  it  contains  the  group 
OH2.OH.  If  it  is  a  secondary  alcohol,  it  contains  the 
group  CH.O  H.  And,  if  it  is  a  tertiary  alcohol,  it  con- 
tains the  group  C.OH.  But  these  groups  may  enter  into 
a  great  variety  of  compounds;  and  frequently,  after  we 
have  determined  the  presence  of  one  or  the  other  of  these 
groups,  it  would  still  be  necessary  to  determine  the  con- 
stitution of  the  group  with  which  it  is  combined.  These 
special  determinations  will  be  considered  later. 

Mercaptans. — If  in  place  of  hydroxyl,  in  the  alcohols 
we  have  considered,  we  introduce  the  group  HS,  bodies 
are  obtained  which  have  been  called  mercaptans.  These 
are  in  many  respects  analogous  to  alcohols,  though  in 
I' -"'r  reactions  they  differ  from  them  somewhat.  Their 
constitution  is  the  same  as  that  of  the  alcohols.  The 
principal  method  for  their  formation  consists  in  the  action 
of  potassium  sulphydrate,  KSH,  on  the  chlorides  of 


GENERAL    CONSIDERATIONS.  135 

alcohol  residues.  These  latter  are  obtained  by  replacing 
the  hydroxyl  of  alcohols  by  chlorine,  and  the  reaction  for 
the  formation  of  the  mercaptans  takes  place  as  follows: — 

RC1     +     K— SH     =     R— SH     -j-     KC1. 

Alcoholic  Mercaptan. 

chloride. 

It  will  thus  be  seen  that  the  group  SH  occupies  the  place 
which  was  occupied  by  the  group  OH  in  the  original 
alcohol.  Theoretically,  a  mercaptan  may  be  prepared 
corresponding  to  every  alcohol.  Thus  we  might  have 
primary,  secondary,  and  tertiary  mercaptans,  correspond- 
ing to  all  the  known  primary,  secondary,  and  tertiary 
alcohols.  Only  such  mercaptans  have  been  prepared  up 
to  the  present  as  correspond  to  the  primary  alcohols. 

Acids What  has  been  said  above  concerning  acids  in 

general  is  true  of  the  acids  of  carbon.  They  contain 
hydroxyl,  and  possess  the  properties  which  we  have 
recognized  as  belonging  to  acids.  In  general,  they  are 
weaker  than  other  acids,  though  they  differ  among  each 
other  in  strength  between  comparatively  wide  limits. 
We  have  several  series  of  acids  of  carbon,  corresponding 
to  the  series  of  hydrocarbons  and  alcohols.  The  simplest 
carbon  acid  is  derived  from  methane,  and  has  the  formula 
H 

It  differs  from  the  simplest  alcohol  in 
O=C—  O  —  H 

containing  an  atom  of  oxygen  in  the  place  of  two  atoms 
of  hydrogen.  Just  as  this  alcohol  consists  of  hydrogen 
combined  with  the  group  CH.OH,  so  the  acid  consists  of 
hydrogen  combined  with  the  group  CO. OH.  This  is  the 
characteristic  group  of  the  acids  of  carbon. 

Proofs. — In  the  first  place,  the  presence  of  hydroxyl  is 
proved  the  same  as  in  the  case  of  ordinary  acids.  In  the 
acid  HsCOa,  if  we  assume  the  presence  of  hydroxyl,  we 
have  the  formula  HCO— OH.  Further,  the  other 
hydrogen  atom  contained  in  the  acid  does  not  conduct, 
itself  ,as  if  it  were  in  combination  with  oxygen,  but  1. ' 
same  as  hydrogen  atoms  which  are  in  combination  with 
carbon  direct!}'.  No  changes  which  the  acid  undergoes 
indicate  any  connection  between  this  hydrogen  atom  and 


136  CHEMICAL    COMPOUNDS. 

oxygen  atom,  so  we  may  conclude  that  they  are  not 
present  as  hydroxyl.  But,  if  they  are  not  present  as 
hydroxyl,  they  must  be  united  directly  with  the  carbon 

H 
atom,  and   the  formula  is  .      Now  by 

0=0— O— H 

certain  appropriate  reactions,  it  is  possible  to  replace  that 
hydrogen  atom  in  this  acid  which  is  in  direct  combination 
with  the  carbon  by  groups  such  as  CH:H,  C.^H5,  etc.  The 
compounds  thus  obtained  must  contain  the  group  CO.O  H. 
They  possess  all  the  properties  of  acids. 

Methods  for  the  Formation  of  the  Acids  of  Carbon. — 
The  methods  of  preparation  of  the  acids  of  carbon  enable 
us  also  to  judge  of  their  constitution.  Some  of  these 
methods  may  be  briefly  described. 

1.  The  simplest  acid,  above  referred  to,  viz.,  H2COV,  is 
'obtained  by  bringing  carbon  monoxide,  CO,  together  with 
potassium  hydroxide,  KO  hi.     The  two  substances  com- 
bine directly,  yielding  the  potassium  salt  of  the  above 
acid,  thus: — 

CO     +     KOH  HC02K. 

From  this  experiment  we  conclude  that,  in  the  salt,  one 
of  the  oxygen  atoms  is  in  direct  combination  with  carbon, 
as  it  was  in  carbon  monoxide,  while  the  other  oxygen 
atom  serves  the  purpose  of  linking  the  carbon  atom  to 
the  potassium.  Hence  the  group — 

COOK    or    O=C— O— K 

I 
is  present  in  the  salt,  and  the  group — 

O=C— O— H  in  the  acid. 

I 

2.  When  methane,  CH4,  is  allowed  to  act  upon  carbonyl 
chloride,  COC1  ,  one  of  the  chlorine  atoms  is  replaced  by 
the  residue  CH3,  thus: — 

(I.)     CfJ4     -f     COCI2    ==    CH3.COC1     +     HC1. 

When  the  product  is  treated  with  water,  the  second 
chlorine  atom  is  replaced  by  OH,  as  follows: — 

(II.)     CB..COC1    -f    HHO    =  -CH8.CO.OH    +    HC1. 


GENERAL    CONSIDERATIONS.  137 

Now  carbonyl  chloride  is  obtained  by  the  direct  addition 
of  C12  to  carbon  monoxide,  CO  ;  and  hence  must  have 

the  constitution  .       The   simplest    interpre- 

Ci 

tation  of  reaction  (I.)  above  is  that  the  residue  CH3  takes 
the  place  occupied  by  one  of  the  chlorine  atoms,  which 

H3C— C=O 
would  give    •  j  .      Lastly,  the  simplest  inter- 

01 

p fetation  of  reaction  (II.)  is  that  the  hydroxyl  group 
enters  into  the  place  of  the  second  chlorine  atom,  which 

H3C— C=0 

gives  as  the  constitution  of  the  product 


This  product  is  acetic  acid,  a  homologue  of  the  simplest 
acid  of  carbon.  It  contains  the  group  CO.OH. 

CN 
3.  The  compound  cyanogen,     I       ,    is  converted  into 

CN 

an  acid  by  the  action  of  water.  This  acid  has  the  for- 
mula C.,H2O4.  It  is  a  bibasic  acid,  and  hence  contains 
two  hydroxyl  groups,  which  would  lead  to  the  formula 
C202(OH)2.  As  both  the  hydroxyl  groups  conduct  them- 
selves in  exactly  the  same  way,  it  is  concluded  that  they 
are  combined  in  exactly  the  same  way.  The  only  for- 

</o~H 

mula  that  satisfies  these  conditions  is      I  or 


CO.OH 

In  this  compound  we  have  two  groups 
CO.OH 

CO.OH,  and,  as  we  have  seen,  it  is  a  bibasic  acid.     There 

12* 


138  CHEMICAL    COMPOUNDS. 

are  a  groat  many  compounds  containing  the  group  CN 
acting  as  a  univalent  group.  By  treating  these  with 
solutions  of  metallic  hydroxides,  the  nitrogen  is  given 
off  in  the  form  of  ammonia,  NH3,  and  in  its  place  are 
taken  up  two  atoms  of  oxygen  and  an  atom  of  a  uni- 
valent element.  The  group  with  which  the  CN  is  in 
combination  remains  unchanged.  Hence,  in  accordance 
with  the  above  experiment,  it  is  believed  that  this  reac- 
tion consists  in  a  con  version  of  the  group  CN  into  CO  OH 
or  COOM,  in  which  M  represents  one  atom  of  a  univa- 
lent metal.  The  constitution  of  this  group  is,  of  course, 
expressed  as  above  by  the  formula  0=C — O — M.  All 

I 

the  substances  thus  prepared  and  containing  this  group 
are  derivatives  of  the  acids ;  they  are  salts. 

4.  The  acids  are  derived  from  primary  alcohols  by 
simple  oxidation.  When  an  acid  is  obtained  from  either 
a  secondary  or  a  tertiary  alcohol,  a  complex  oxidation 
takes  place ;  the  molecule  must  be  broken  up  and  re- 
arranged. We  have  seen  that  the  difference  between  the 
three  classes  of  alcohols  is  due  to  the  difference  in  the 
characteristic  groups  of  each  class;  these  being  CH2OH, 
CHOH,  and  COH.  The  oxidation  consists  in  the  ab- 
straction of  hydrogen  and  addition  of  oxygen.  If  hydro- 
gen is  abstracted  from  and  oxygen  added  to  the  group 

I 
CH2OH,  we   obtain    COOH  or   O=C— O—  H.     If  we 

abstract  hydrogen  from  and  add  oxygen  to  the  group 
CHOH,  which  is  bivalent,  we  could  not  obtain  another 
bivalent  group,  unless  it  be  one  with  an  exceedingly 

>° 

improbable  formula  as,  for  instance,       — C<^        ,      and 

|\0 

this  would  contain  no  Irydrogen,  so  that  it  could  not  be 
considered  as  characteristic  of  acids.  Lastly,  the  group 
COH  cannot  lose  hydrogen  and  gain  oxygen  and  still 
remain  trivalent.  As  then  the  only  alcohols  which  can 
yield  the  group  COOH  by  the  oxidation  of  their  own 
characteristic  group  are  the  primary  alcohols,  and  as 
those  alcohols  which  cannot  yield  this  group  by  such 
oxidation  do  not  yield  corresponding  acids,  the  con- 


GENERAL    CONSIDERATIONS.  139 

elusion  is  drawn  that  the  change  which  takes  place  when 
primary  alcohols  are  oxidized  consists  in  the  conversion 
H 


of  the  group     —  C—  0—  H      into  0=C—  0—  H, 


which 


is  a. necessary  constituent  of  carbon  acids. 

Aldehydes. — Aldehydes  are  products  derived  from  the 
partial  oxidation  of  primary  alcohols,  the  group  CH2OH 
being  converted  into  COH.  This  group  is  not  identical 

with  the  group      — C — 0 — H    of  tertiary  alcohols,  but 

1 

has  the  constitution  expressed  by  the  formula  — C— O  . 
It  is  a  univalent  group,  just  as  the  group  CH2OH,  from 
which  it  is  derived,  is  univalent;  whereas,  the  tertiary 
alcohol  group,  COH,  is  trivalent.  The  aldehydes  are 
intermediate  products  between  primary  alcohols  and  the 
acids  which  these  yield.  It  was  shown  that  the  acids 
are  formed  from  these  alcohols  by  the  extraction  of 
hydrogen  and  addition  of  oxygen.  If  hydrogen  is  only 
abstracted  and  no  oxygen  added,  the  product  is  an  alde- 
hyde, thus: — 

R— CH2OH,       R— COH,        R— COOH. 

Primary  alcohol.  Aldehyde.  Acid. 

Proofs. — The  proofs  of  the  general  constitution  of 
aldehydes  are  similar  to  those  given  for  the  acids.  Take, 
for  instance,  the  simplest  aldehyde.  This  has  the  for- 
mula H2CO.  The  tests  for  the  presence  of  hydroxyl, 
above  considered,  if  applied  to  aldehydes,  show  that  the 
group  is  not  present.  On  the  contrary,  if  the  aldehydes 
be  treated  with  the  chloride  of  phosphorus,  the  oxygen 
atom  is  extracted  and  its  place  is  taken  by  two  chlorine 
atoms.  This  shows  that  the  oxygen  was  held  in  combi- 
nation by  two  affinities  of  the  carbon  atom,  and,  conse- 
quently, it  could  not  have  been  present  in  combination 
with  hydrogen,  forming  hydroxyl.  We  are  thus  led  to 


140  CHEMICAL    COMPOUNDS. 

H 

the  formula      0=0  —  H     for  the  above  compound.     It 
consists  of  a  hj'drogen  atom  combined  with  the  group 

/H 

—  C=O  .  Other  aldehydes  are  derived  from  this  sim- 
plest one  by  replacing  one  of  the  hydrogen  atoms  with  a 
residue  of  greater  or  less  complexity.  Thus,  we  may  intro- 
duce the  group  C  H3  or  C2H5,  and  we  would  obtain  the  com- 
pounds CH3  —  COH  and  C2H5  —  OOH  respectively,  both  of 
which  are  aldehydes. 

The  methods  for  the  preparation  of  aldehydes  also  fur- 
nish proof  of  the  constitution  above  ascribed  to  them. 
Some  of  these  are  the  following:  — 

1.  We  have  already  seen,  that,  when  acids  are  treated 
with  the  chlorides  of  phosphorus,  their  hydroxyl  is 
replaced  by  an  atom  of  chlorine,  yielding  chlorides  of 
acid  residues.  Each  such  chloride,  as  was  shown,  con- 


tains  the  group     —  0=0    .      If  we  could  replace  the 
chlorine   atom    in   this   group    by  hydrogen,  we    would 

/H 

plainly  have  the  characteristic  aldehyde  group   —  0=0  . 
Such  a  replacement   has    been  effected  in  the  cases  of 
some  of  the   chlorides,  and   the  resulting   bodies  were 
found  to  be  the  expected  aldehydes. 

2.  When  a  salt  of  any  acid  of  carbon  is  mixed  with  a 
salt  of  the  simplest  acid  of  carbon  (formic  acid),  of  the 
formula  H.CO.OH,  and  the  'mixture  distilled,  an  alde- 
hyde is  obtained  together  with  a  carbonate.  The  carbon- 
ates are  derived  from  a  bibasic  acid,  and  have  the  formula 


, 

0=0      .       It  seems  rational  to  suppose  that  the  groups 


OM  have  passed  directly  from  the  compounds  in  which 
they  were  originally  contained  to  the  carbonate,  and  that 
the  group  CO  also  has  been  derived  directly  from  one  of 
the  original  acids.  If  these  suppositions  are  correct, 


GENERAL    CONSIDERATIONS.  141 

then  we  are  led  to  the  conclusion  that  the  aldehyde 
resulting  from  the  described  reaction  contains  the  group 

/H 

— C=O  .  For,  let  R — CO.OM  represent  the  formula 
of  any  salt  of  a  carbon  acid,  and  H.CO.OM  a  salt  of 
formic  acid.  On  bringing  these  two  compounds  together 
and  heating  them,  either  one  of  two  things  can  take 
place  if  the  above  suppositions  are  correct.  The  groups 
forming  the  carbonate  may  be  split  off  thus : — 

R_[~CQHOM" 

H—  CO—  OM 
or,  thus : — 

R— CO— |OM 

H— 'CO— OM 


The  remaining  groups,  uniting  in  the  simplest  way,  will 

> 

give  n  s,  in  the  first  place,  a  compound,  R  —  C=O  , 
and,  in  the  second  place,  a  compound  of  exactly  the 
same  constitution. 

3.  Aldehydes  are  formed  only  from  primary  alcohols, 
not  from  secondary  or  tertiary  alcohols.  If  we  examine 
the  group  characteristic  of  the  alcohols,  we  shall  find  that 
the  only  one  of  them  which  is  capable  of  transformation 

/H 

into  the  group    —  C=O    is   that   of  primary   alcohols, 

/H 

—  CJETjOH;  and,  further,  the  group  —  C=O  is  the  only 
one  containing  carbon,  hydrogen,  and  oxygen  in  the  same 
proportions  which  can  be  derived  from  the  group  of 
primary  alcohols,  and  not  from  those  of  secondary  and 
tertiary  alcohols. 

These   considerations    make   it   exceedingly   probable 


that  all  aldehydes  contain  the  group  —  C=0  ,  as  above 
stated. 


142  CHEMICAL    COMPOUNDS. 

Acetones.  —  Acetones    are    products    of   the    partial 
oxidation   of  secondary  alcohols,  the  group    —  C—  OH 

H 
being  converted  into  the  group     C=O  .         The     alde- 

hydes, too,  contain  the  group     C^O  ;     but  it  is  further 

characteristic  of  aldehydes  that  one  of  the  affinities  of 
this  group  is  saturated  with  hydrogen,  giving  the  com- 

plete group     C=O  .       On  the  other  hand,  it  is  charac- 

H 

teristic  of  acetones  that  both  of  the  affinities  of  the  group 

C=0     are  saturated  with  hydrocarbon  residues.     Thus 


the  simplest  acetone  known  has  the  formula 


3 
0=0   , 


CET3 

both  the  affinities  of  the  characteristic  group  being  satu- 
rated with  residues  of  the  hydrocarbon  methane,  CH4. 

* 

Proofs.  —  As  just  stated,  the  simplest  acetone  has  the 
formula  C3H6O.  If  a  chloride  of  phosphorus  be  allowed 
to  act  upon  this  compound,  the  result  is  similar  to  that 
obtained  in  the  same  experiment  with  aldehydes,  viz.,  the 
atom  of  oxygen  is  abstracted,  and  two  chlorine  atoms 
take  its  place.  This  shows  that  the  oxygen  was  not 
present  as  hydroxyl,  but  was  combined  with  the  carbon 
atom  by  means  of  two  affinities,  forming  the  group 

(U. 


GENERAL    CONSIDERATIONS.  143 

Again,  if  nascent  hydrogen  is  allowed  to  act  upon  this 
acetone,  secondary  propyl  alcohol  is  the  product,  and  the 

CH3 
I  /OH 

alcohol  has  the  formula     C-S  .       From    this    we 

|\H 
CH3 

conclude  that  in  acetone,  as  well  as  in  secondary  propyl 
alcohol,  the  two  groups  CH3  are  present;  and  we  are 

CH3 

thus  led  to  the  formula     C=0     for  the  simplest  ace- 

CH3 

tone.  We  recognize  in  this  formula  what  we  have  stated 
to  be  the  characteristic  of  all  acetones,  viz.,  it  consists  of 
two  hydrocarbon  residues  combined  by  means  of  the 

bivalent  group     C=O   . 

The  following  methods  of  preparation  serve  as  proofs 
of  the  accepted  constitution  of  acetones: — 

1.  Just  as  aldehydes  are  obtained  from  acid  chlorides 
by  replacing  the  chlorine  with  hydrogen,  so  acetones  are 
obtained  from  the  same  chlorides  by  replacing  the  chlo- 
rine with  hydrocarbon  residues.  By  treating  acetyl 
chloride,  C,H3O.C1,  with  zinc  methyl,  Zn(CH3)2,  ordinary 
acetone,  CO(CH3)2,  is  produced,  together  with  zinc  chlo- 
ride, ZnCl2.  The  formula  of  acetyl  chloride  is  known  to  be 

CH3 — C=0  .  Hence  the  simplest  interpretation  of  the 
above-  reaction  is  that  a  methyl  group  of  zinc  methyl 
takes  the  place  of  a  chlorine  atom  in  acetyl  chloride, 
thus: — 

/CH3         CH-C-CH, 

+  ZnCl.. 
CH3— C— CH3 

\ 


144  CHEMICAL    COMPOUNDS. 

And  this  leads  us  clearly  to  the  formula  above  assumed 
as  representing  the  constitution  of  acetone. 

2.  When  the  salts  of  many  acids  of  carbon  are  sub- 
jected to  dry  distillation,  acetones  are  formed,  together 
with  a  carbonate  or  carbonates.  This  reaction  is  analo- 
gous to  the  reaction  for  the  preparation  of  aldehydes,  loy 
the  distillation  of  a  mixture  of  the  salt  of  some  carbon 
acid  and  a  salt  of  formic  acid.  What  was  said  in  regard 

to  the  latter  reaction,  showing  that  the  group  C=O  must 

be  present  in  aldehydes,  holds  good  in  regard  to  the  re- 
action under  consideration,  and  shows  just  as  conclusively 

I 

that  the  group  C=0  must  be  present  in  acetones.     Let 

I 

R.CO.OM  represent  a  salt  of  an  acid  of  carbon.  Its 
decomposition  by  heat  may  be  represented  as  follows: — 

B.JCO.OM] 

R.CO.IOM 

The  residues  uniting,  we  have  a  compound,  R — CO — R, 
which  has  the  general  formula  of  an  acetone,  as  above 
assumed.  Or  let  R.COOM  represent  the  salt  of  one 
carbon  acid  and  R'.COOM  the  salt  of  another  carbon 
acid,  in  which  R  and  R'  are  both  hydrocarbon  residues. 
The  decomposition,  which  takes  place  when  a  mixture  of 
these  two  salts  is  heated,  is  represented  as  follows: — 


R.I  COO  M 
R'.CO|OM 

•_ 

This  gives  a  compound  of  the  formula  R — CO — R'. 

It  will  be  seen  that  one  of  the  first  conditions  for  the 
production  of  an  acetone  by  means  of  this  reaction  is 
that  neither  of  the  salts  employed  be  a  formate,  H.COOM, 
as  the  use  of  the  latter  would  give  rise  to  the  formation 
of  an  aldelryde. 

3.  Acetones  are  produced  by  the  partial  oxidation  of 
secondary  alcohols.  Considerations,  similar  to  those 


GENERAL    CONSIDERATIONS.  145 

employed  in  the  cases  of  acids  and  aldehydes,  show  that 
the  supposition,  that  the  group  CO  is  present  in  acetones, 

I 

is  most  in  harmony  with  the  fact  of  the  ready  transform- 
ation of  secondary  alcohols  into  acetones. 

Ethers.  —  When  acids  and  bases,  in  general  terms,  act 
upon  each  other,  salts  are  formed,  water  being  eliminated. 
Just  so  when  alcohols  and  carbon  acids  act  upon  each 
other,  bodies,  similar  to  salts,  are  formed,  water  being 
eliminated  :  — 

H.COOH  +    C2H5.OH    =    H.CO.OC,!!.   +   H2O  ; 

Formic  acid.  Alcohol.  New  body. 

/OH  /O.CH3 

S02<  +  2CH3.OH     =     S02<  4-  2HaO  . 


Sulphuric  acid.  Methyl  alcohol.  New  body. 

NO—  OH    +    C2H5—  OH     =    N03—  OC2H5    +   H20. 

Nitric  acid.  Alcohol.  New  body. 

It  will  be  seen  that  these  bodies  differ  from  salts  in 
that  they  contain  hydrocarbon  residues  in  the  place  of 
metals.  Salts  were  denned  as  acids  in  which  the  hydro- 
gen of  the  hydroxyl  group  is  replaced  by  a  base  residue. 
These  bodies  are  acids  in  which  the  hydrogen  of  the 
hydroxyl  group  is  replaced  by  a  hydrocarbon  residue. 
All  bodies  of  this  kind  are  called  ethers,  and  to  distin- 
guish them  from  another  class  of  bodies  known  as  simple 
ethers,  and  which  will  be  considered  below,  they  are  usually 
known  as  compound  ethers.  The  analogy  between  com- 
pound ethers  and  salts  is  very  close,  and  hence,  if  the 
nature  of  salts  is  understood,  the  nature  of  these  ethers 
will  also  be  readily  understood.  We  may  have  compound 
ethers  derived  from  monobasic,  bibasic,  tri  basic,  etc. 
acids,  and  we  may  have  compound  ethers  containing  uni- 
valent,  bivalent,  trivalent,  etc.  hydrocarbon  residues  ;  as, 
for  instance, 

Ethers  from  monobasic  acids  :  — 

NO,.O.C,H5,         CH3.CO.O.CH3,        etc. 

Ethyl  nitrate.  Methyl  acetate. 

13 


146  CHEMICAL    COMPOUNDS. 

Ethers  from  bibasic  acids: — 

/CO.O.CjHa 

etc. 


,O.CH3  /CO.O.C,!^ 


XO.CH3  \CO.O.CaH5 

Methyl  sulphate.  Ethyl  succiuate. 

Ethers  from  tribasic  acids  — 

,O.C2H5  ,CO.O.C,H5 

PO— O.C2H5    ,  C3H5— CO.O.C2H5   ,        etc. 

\).C3H5  \CO.O.C,H. 

Ethyl  phosphate.  Ethyl  tricarballylnte. 

The  above  ethers  all  contain  univalent  hydrocarbon 
residues.  Among  those  containing  bivalent  residues  may 
be  mentioned — 

CHb.CO.CK 

CH:rCO.OX 

Ethylene  diacetate. 

Proofs. — The  fact  that  compound  ethers  are,  in  many 
cases,  formed  by  the  direct  action  of  acids  upon  alcohols, 
and  that  water  is  formed  at  the  same  time,  taken  together 
with  our  knowledge  concerning  the  constitution  of  acids 
and  alcohols,  points  clearly  to  the  constitution  for  these 
ethers  which  has  been  above  assumed.  But  another 
method  of  formation  is  just  as  decisive  in  its  testimony. 

If  the  silver  salts  of  acids  are  treated  with  the  chlo- 
rides, bromides,  or  iodides  of  hydrocarbon  residues,  com- 
pound ethers  are  formed  in  which  the  hydrocarbon  resi- 
dues are  found  in  the  place  of  the  silver  which  was  in 
the  salts,  and  the  silver  itself  is  found  in  combination 
with  the  chlorine,  bromine,  or  iodine  which  was  in  com- 
bination with  the  h}^drocarbon  residues.  This  is  seen  in 
the  following  typical  reactions : — 

CH3.CO.OAg    +    C,H5I    =     CH3.CO.O.C2H5    +   Agl. 

Silver  acetate.  Ethyl  iodide.  Ethyl  acetate. 

/CO.OAg  /CO.OCH3 

CTT  /  !O/riTTT\  P  TT  /  J_   9  A  o-r 

2±lA  -f-   A^Ha1)   =  ^2M4\  H-  ^Ag'- 

xCO.OAg  \CO.OCH3 

Silver  succiuate.  Methyl  iodide.  Methyl  succinate. 

'  Simple  Ethers. — Simple  ethers  correspond  to  the 
metallic  oxides.  They  consist  of  two  hydrocarbon  resi- 


CH. 
>0  , 
CH/ 

Methyl  ether. 

°'H>  , 

CH/ 

Methyl-ethyl  ether. 

CH> 

Ethyl  ether. 

GENERAL    CONSIDERATIONS.  147 

dues  united  by  means  of  an  oxygen  atom,  just  as  the 
metallic  oxides  consist  of  two  base  residues  united  by 
means  of  an  oxygen  atom.  Examples  of  these  are  the 
folio  wins: : — 


etc. 


Proofs. — The  constitution  of  these  compounds  is  ren- 
dered clear  by  a  consideration  of  one  of  the  principal 
methods  of  their  formation. 

When  an  alcohol  is  treated  with  sodium  or  potassium, 
as  we  have  seen,  the  hydrogen  of  the  hydroxyl  is  replaced 
by  the  metal  emplo3-ed.  We  thus  obtain  compounds 
such  as  sodium  ethylate  CaH5.ONa,  sodium  methylate 
CET.ONa,  etc.  If  these  compounds  are  further  treated 
with  the  iodides  of  hydrocarbon  residues,  the  iodine  com- 
bines with  the  metal  and  the  residues  unite.  Thus  we 
have  the  following  reactions : — 

C3H5.ONa    +    C3H5I      =      C2H5-0-C2H5     +    Nal, 

Sodium  ethylate.        Ethyl  iodide.  Ethyl  ether. 

CH3.ONa     +   CHSI      =     CH:?— 0— CH3     +      Nal, 

Sodium  methylate.     Methyl  iodide.  Meihyl  ether. 

CH3.ONa  -f    C2H.I        =       CH— O— C2H5    +  Nal, 

Sodium  methylate.     Ethyl  iodide.  Methyl-ethyl  ether. 

C2H..ONa     +     CH3I       =      C2H5— 0— CH3   +    Nal. 

Sodium  ethylate.          Methyl  iodide.  Methyl-ethyl  ether. 

From  these  reactions  we  see  what  the  constitution  of 
the  ethers  formed  jnust  be.  We  are  in  each  case  justified 
in  assuming  that  the  hydrocarbon  residue  enters  into  the 
new  compound  in  the  place  occupied  by  the  metal,  and, 
according  to  our  conceptions  concerning  alcohols,  this 
metal  is  united  to  the  rest  of  the  molecule  in  which  it 
is  contained  by  means  of  an  oxygen  atom. 

Anhydrides. — The  anhydrides  of  carbon  compounds 
are  derived  from  carbon  acids  in  the  same  way  that  an- 
hydrides in  general  are  derived  from  acids ;  and  all  the 
possibilities  which  we  considered  above  hold  good  for 
these  anhydrides.  There  are  anhydrides  derived  from 
monobasic,  bibasic,  tribasic  acids,  etc.  There  are  partial 


148  CHEMICAL    COMPOUNDS. 

and  complete  anhydrides:  but,  further,  there  are  anhy- 
drides derived  from  compounds  which  partake  of  the 
doable  character  of  alcohol  and  acid.  In  these  com- 
pounds the  hydroxyl  which  imparts  the  alcoholic  cha- 
racter and  that  which  imparts  the  acid  character,  both 
together  furnish  the  elements  which  form  the  water 
given  off. 

Peculiar  Anhydrides.  —  Lactic    acid    has    the  formula 

/OR 

CH  .CH/  .      It  contains  an  alcoholic  hydroxyl 

\COOH 

and  an  acid  hydroxyl.     When  the  acid  is  distilled  it  loses 

-0-, 
water,  and  a  compound  of  the  formula    CIL.CH 


is  formed.  This  is  lactic  anhydride.  As  will  be  seen, 
the  anhydride  is  formed  by  the  loss  of  the  elements  of 
water  from  both  hydroxyl  groups  together.  Salicylic 


, 

acid   has   the  formula      C6H4\  •      It   forms   an 

\COOH 
anhydride   in  a  similar    manner,  having  the    formula  — 

-0- 


SUBSTITUTION  PRODUCTS.  ' 

We  have  thus  far  considered  the  various  classes  of 
chemical  compounds  which  are  known  to  exist,  and  have 
shown  that  each  class  is  characterized  by  some  peculiarity 
of  constitution  which  we  recognize  in  each  member  of  the 
class.  There  is  in  each  compound  a  group  which  deter- 
mines its  character,  making  it  an  acid  or  an  alcohol,  an 
acetone  or  an  aldehyde,  etc.  As  long  as  this  group  re- 
mains unchanged,  the  compound  belongs  to  the  same 
class.  If  the  group  is  changed,  the  compound  loses  its 


GENERAL    CONSIDERATIONS.  149 

characteristics,  and  belongs  to  another  class.  On  the 
other  hand,  the  residues  with  which  the  class  groups  are 
united  may  undergo  a  variety  of  changes  without  inter- 
fering at  all  with  the  general  properties  of  the  compounds. 
The  most  common  of  these  changes  are  those  which  are 
effected  by  substitution. 

Chemical  compounds  act  upon  each  other,  in  general, 
in  two  ways.  1st.  They  unite  directly,  forming  only  one 
product,  as  we  see  in  the  following  reactions:— 


+         HC1         = 

Ammonia.  Ammonium  chloride. 

C2H4         +         Br2        **         C2H4Br2 

Ethylene.  Ethylene  bromide. 

2d.    They  exchange  certain  constituents,  forming  two  or 
more  new  products,  thus:  — 

C2H6       +       C12      =       C2H5C1       +       01 

Ethane.  Chlorethane. 

CH8.COH     +     6C1     =     CCla.COH     +     3HC1 

Aldehyde.  Tiichloraldehyde. 

C6H6       +       H,S04      =*      C6H6.Sq,H       +       H2O 

Benzene.  Sulphobenzenic  acid. 

C6H6     +.-HNO,    =    O.H,(NOJ     +    H,O  •  .  , 

Benzene.  Nitrobenzene. 

The  latter  kind  of  action  is  by  far  the  most  common. 
It  is  that  which  is  called  substitution.  In  the  above 
examples,  the  principal  products  are  called  substitution 
products,  though,  strictly  speaking,  both  products  are 
substitution  products. 

While  the  principle  of  substitution  is  recognized  in. 
connection  with  nearly  all  chemical  reactions,  and  hence 
nearly  all  chemical  compounds  may  be  considered  as 
substitution  products  with  reference  to  some  other  com- 
pounds, still  it  is  customary  to  include  under  this  head 
only  those  products  which  are  formed  by  the  replacement 
of  hydrogen  in  carbon  compounds;  and  the  substitutions 
which  are  spoken  of  are  only  those  which  can  be  actually 
effected  —  not  imaginary  cases. 

Substitution  Products  containing  Chlorine,  Bromine, 
or  Iodine.  —  The  simplest  examples  of  substitution  pro- 
ducts are  those  which  are  formed  by  the  action  of  any  of 

13* 


150  CHEMICAL    COMPOUNDS. 

the  so-called  haloids  (01,  Br,  T)  upon  carbon  compounds. 
The  action  consists  in  the  abstraction  of  one  or  more 
atoms  of  hydrogen  from  the  compound,  and  the  filling  of 
the  places  left  vacant  by  a  corresponding  number  of 
atoms  of  the  substituting  element.  The  constitution  of 
the  products  is  the  same  as  that  of  the  compounds  from 
which  they  are  derived.  Thus  we  have  acetic  acid, 
CH.^.CO.OH;  if  chlorine  acts  upon  it,  the  following 
reactions  take  place  successively: — 

CH3.CO.OH  -f  C12  =  CH2C1.CO.OH  -f  HOI. 
CH2C1.CO.OH  +  C12  =  CHC1.CO.OH  -f  HC1. 
CHC12.CO.OH  •  +  C12  =  CC13.CO.OH  +  HC1. 

The  constitution  of  these  three  different  products  is 
essentially  the  same  as  that  of  the  acid  from  which  they 
are  derived. 

j\mong  these  simple  substitution  products,  however, 
differences  are  possible,  and  are  actually  observed,  which 
are  not  possible  in  the  original  compounds.  Take  the 
compound  propane,  C3H8.  The  constitution  of  this  hj'rtro- 

H    H 


carbon  is     H — C — C — 0 — H  .        The  hydrogen  atoms 


cannot  be  arranged  in  any  other  way  with  reference  to 
the  carbon  atoms.  There  is  only  one  hydrocarbon  of 
this  composition  possible.  But  the  carbon  atoms  in  this 
compound  differ  from  each  other.  The  two  which  are 
represented  as  ending  the  chain  in  the  formula  are  alike, 
while  the  central  atom  differs  from  them.  The  first  are 
in  combination  with  carbon  by  means  of  only  one  affinity 
each,  while  the  central  atom  is  joined  to  carbon  by  means 
of  two  affinities.  We  would  naturally  expect  then  that 
the  difference  between  these  two  kinds  of  carbon  atoms 
would  cause  a  difference  between  the  hydrogen  atoms 
combined  with  them.  If  such  a  difference  exists,  then, 
different  products  must  be  obtained  according  as  we 
replace  a  hydrogen  atom  attached  to  one  of  the  terminal 
carbon  atoms,  or  another  hydrogen  atom  attached  to  the 
central  carbon  atom.  Thus,  if  in  the  following  formula — 


GENERAL    CONSIDERATIONS.  151 

7 


'?  T  r 

1     TT          p p p 

A    £1         l_y         v>         \j 


H6 


3  H    H    H  5 

8 

we  replace  any  of  the  hydrogen  atoms  numbered  1,  2,  3, 
4,  5,  6  by  an  element  such  as  chlorine,  the  resulting  com- 
pound would  in  each  case  be  the  same. 

If,  however,  we  replace  one  .of  the  hydrogen  atoms 
numered  1  or  8  by  the  same  element,  a  compound  of  the 
same  composition,  but  of  different  constitution,  would  be 
obtained.  The  formulas  of  the  two  compounds  would  be 
respectively 

CH2C1CH2.CH3    and     CH:J.CHC1.0H3. 

Thus  we  see  that  the  position  of  a  substituting  element 
must  be  taken  into  consideration  in  studying  the  consti- 
tution of  compounds.  In  connection  with  the  individual 
compounds,  which  will  be  briefly  considered  in  the  last 
section  of  this  book,  the  methods  will  be  described  which 
enable  us  to  determine  the  positions  of  substituting  ele- 
ments and  groups. 

Complex  Substitution  Products. — Under  this  head  we 
include  all  those  products  which  are  formed  by  replacing 
the  hydrogen  of  a  carbon  compound  either  partially  or 
wholly  by  groups.  In  accordance  with  what  has  just 
been  said  concerning  the  simple  substitution  products,  it 
is  plain  that,  in  studying  the  constitution  of  the  complex 
substitution  products,  two  things  must  be  taken  into 
consideration : — 

1st.  The  constitution  of  the  substituting  group  itself, 
and, 

2d.  The  position  of  the  group  in  the  molecule  of  the 
substitution  product. 

We  shall  here  only  take  up  the  first  part  of  the  problem. 

Constitution  of  Substituting  Groups.  —  The  groups 
which  we  shall  have  to  consider  are  the  following :  The 
cyanogen  group  ON,  and  an  isorneric  group;  the  sulpho 
group  S03H ;  the  nitro  group  N0.2;  the  nitroso  group 


152  CHEMICAL    COMPOUNDS. 

NO;  the  amid  o  group  NH2;  the  imido  group  NH ;  and 
a  few  other  groups  intimately  connected  with  those 
mentioned. 

Constitution  of  the  Group  CN. — That  acid  of  carbon 
which  consists  of  a  nitrogen  atom  and  a  hydrogen  atom 
combined  with  a  carbon  atom,  viz.,  hydrocyanic  acid,  has 
already  been  referred  to.  Ity  appropriate  reactions  it 
is  possible  to  transfer  the  group  ON,  contained  in  hydro- 
cyanic acid,  to  other  compounds  in  such  a  way  that  it 
takes  the  place  of  hydrogen,  forming  a  substitution  pro- 
duct. It  is  univalent,  and  hence  its  constitution  is 
expressed  by  the  formula  — C=N.  We  have  the  follow- 
ing reactions: — 

CH2C1.COOH  -f  KCN     =    CH2(CN).COOH  +  KC1 

Monocbloracetic  Potassium  Cyanacetic  acid, 

acid.  cyanide. 

C2H4Br2    -f     2KCN       =       CVH4(CN)2    +    2KC1. 

Ethylene  bromide.    Potassium  cyanide.         Ethylene  cyanide. 

These  substitution  products,  which  consist  only  of  the 
group  — C~N  combined  with  a  hydrocarbon  residue,  are 
called  nitriles. 

A  few  other  compounds  are  known  which  have  the 
same  composition  as  the  nitriles,  but  a  different  consti- 
tution. They  are  known  as  isonitriles  or  carbylamines. 
They  contain  the  group  C==N — .  This  group  is  univa- 
lent, just  as  the  group  — (feN,  but  the  nitrogen  atom 
contained  in  it  plays  the  part  of  a  quinquivalent  element, 
whereas,  the  nitrogen  atom  of  the  group  — C=N  is  tri- 
valent.  We  maj^  have  thus  the  two  compounds — 

C2H5-~CzEN  and  C=N— C2H5, 

of  the  same  composition,  but  different  constitution.  Both 
these  compounds  are  well  known.  The  former  is  called 
ethylcyanide  or  propionitrile,  the  latter  ethylcarbylamine. 
As  ethyl  cyanide,  when  treated  with  an  alkali,  yields 
propionic"acid,  we  conclude  that  the  carbon  atom  of  the 
group  CN  is  united  directly  with  the  hydrocarbon  residue. 
For  if  it  had  not  been,  the  removal  of  the  nitrogen  ought 
to  have  caused  the  formation  of  a  product  containing  a 
smaller  number  of  carbon  atoms  than  the  cyanide  itself. 
The  reaction  which  does  actually  take  place  is  that  which 


GENERAL    CONSIDERATIONS.  153 

we  have  considered  above  as  giving  rise  to  the  formation 
of  acids  from  the  cyanides,  viz.: — 

C,H5.CN      +     2HaO       =      C,H5.COOH     +     NH3 

Ethyl  cyanide.  Propiouic  acid. 

If  the  group  CN  had  been  in  combination  with  the 
hydrocarbon  residue  by  means  of  the  nitrogen  atom, 
which  would  be  the  case  if  the  group  had  the  constitution 
Cj=iN — ,  we  should  expect  the  nitrogen  atom  to  remain 
in  combination  with  the  hydrocarbon  residue,  in  case  of 
decomposition,  or  we  should  expect  the  nitrogen  atom  to 
take  with  it  the  carbon  atom  with  which  it  is  most  inti- 
mately combined.  In  either  case,  a  separation  of  the 
carbon  atoms  would  be  the  result,  and  we  would  obtain 
products  containing  a  smaller  number  of  carbon  atoms 
than  the  original  compound  contained.  This  is  exactly 
what  takes  place  when  the  carbylamines  are  decomposed. 
When  treated  with  hydrochloric  acid  they  yield  two  pro- 
ducts ;  one  of  these  is  formic  acid,  a  compound  contain- 
ing one  atom  of  carbon ;  the  other  consists  of  the  hydro- 
carbon residue  of  the  original. compound  combined  with 
the  nitrogen  atom  and  hydrogen.  Thus,  in  the  case  of 
ethylcarbylamine,  the  decomposition  may  be  represented 
as  follows : — 

C,H5) 

CM.—X^C     +     2HTO     =         H  (  N     +     H.COOH 

H\ 

Ethylcarbylamiue.  Ethylamine.  Formic  acid. 

(  C,H5 

The  fact  that  the  compound     N  •<  II        ,  in  which  the 

(H 

nitrogen  atom  is  evidently  in  combination  with  the  hydro- 
carbon residue,  is  so  readily  formed,  leads  us  to  the  con- 
clusion that  in  the  original  compound  the  same  kind  of 
union  existed.  The  fact,  also,  that  the  one  carbon  atom 
is  given  off  so  readily  from  the  molecule,  indicates  clearly 
that  it  was  held  in  combination  in  some  manner  different 
from  that  in  which  the  other  carbon  atoms  of  the  mole- 
cule are  held  in  combination.  Taking  the  two  facts  and 
conclusions  together,  we  are  led  to  the  formula  above 
assigned  to  the  carbylamine  group,  viz.,  C^N —  as  the 
correct  one. 


154  CHEMICAL    COMPOUNDS. 

Constitution  of  the  Group  SO.AH.  —  By  the  action  of 
concentrated*  sulphuric  acid  upon  hydrocarbons  and  vari- 
ous other  compounds  containing  hydrogen,  derivatives 
are  obtained  which  differ  from  the  original  compounds  in 
containing  the  group  SOSH  in  the  place  of  hydrogen. 
The  reaction  consists  in  the  formation  of  water  and  the 
new  derivative,  thus  :  — 

/OH  /C6H5 

o6H9   +   so  /OH         SO<OH    +   n.o  . 

Benzene.  Sulphobenzenic  acid. 

These  products  all  act  like  acids  in  every  way,  so  that 
we  are  justified  in  assuming  the  presence  of  hydroxyl  in 
them.  As  they  are  formed  so  readity  from  sulphuric 
acid,  it  is  also  fair  to  assume  that  the  group  S0.2OH  is  a 
residue  of  sulphuric  acid.  Then,  if  we  know  the  con- 
stitution of  sulphuric  acid,  the  constitution  of  this  group 
will  also  be  known  to  us.  The  fact  that  this  group  is  a 
residue  of  sulphuric  acid  is  shown  also  in  the  following 
way  :  By  replacing  one  of  the  hydroxyl  groups  of  sul- 
phuric acid  by  an  atom  of  chlorine,  we  obtain  a  com- 

/Cl 

pound  of  the  formula     SO./  ,      which,   by    simple 


treatment  with  water,  is  reconverted  into  sulphuric  acid. 
There  can  be  no  doubt  that  the  group  SO2OH  of  this 
chloride  has  exactly  the  same  constitution  as  the  cor- 
responding group  of  the  acid.  But,  if  this  chloride  be 
allowed  to  act  upon  benzene,  sulphobenzenic  acid  and 
hydrochloric  acid  are  the  products,  the  former  having  all 
the  properties  possessed  by  the  sulphobenzenic  acid  ob- 
tained from  the  action  of  sulphuric  acid  upon  benzene. 
The  reaction  takes  place  thus  :  — 

Cl 
C6H6     -f     S02<  =         C6H5.S0.2.OH    +    HC1. 


Here,  evidently,  the  group  SO^.OH  of  the  chloride  takes 
the  place  of  an  atom  of  hydrogen  in  benzene. 

Assuming,  then,  the-  general  formula  SO,  .OH  for  the 
group,  it  remains  to  decide  in  what  manner  the  atoms  of 
the  sub-group  S02  are  united.  If  both  sulphur  and  oxy- 


GENERAL    CONSIDERATIONS.  155 

gen  act  as  bivalent  elements,  two  possibilities  present 
themselves.  We  may  have  either  — O— J3 — 0 —  or 
— S— '0 — 0 — .  If  the  former  expresses  the  constitution 
of  the  group,  then  it  is  plain  that  the  hydrocarbon  must 
be  united  with  it  through  the  instrumentality  of  oxygen, 
whereas,  if  the  latter  is  the  correct  expression,  the  hydro- 
carbon residue  may  be  held  in  combination,  either  through 
the  instrumentality  of  oxygen  or  sulphur.  For,  we  may 
have  either — 

1,  C6H5— 0— 0— S— OH  ;  or,' 2,  C6H,— S— 0— 0— OH. 

Inasmuch  as  by  reduction  sulphyd rates  are  obtained 
from  the  sulpho  acids,  it  is  believed  that,  in  the  latter, 
the  grouping  of  the  atoms  is  similar  to  that  in  the  second 
of  the  above  formulas.  The  sulphydrates  have  the  gene- 
ral formula  R(SH),  in  which  R  represents  a  hydrocarbon 
residue.  This  residue  is  united  directly  with  the  sulphur, 
and  this  in  turn  with  the  hydrogen.  The  proof  of  this 
constitution  of  the  sulphydrates  has  been  considered 
under  the  head  of  hydroxides. 

Whenever  then  the  group  — S — 0 — 0 — OH  enters 
into  a  compound  in  the  place  of  a  hydrogen  atom,  the 
resulting  compound  is  a  true  sulpho  acid. 

Again,  when  a  chloride,  bromide,  or  iodide  of  a  hydro- 
carbon or  other  residue  is  treated  with  an  aqueous  solu- 
tion of  a  neutral  sulphite,  a  derivative  is  obtained  which 
has  the  same  general  composition  as  the  sulpho  acid,  as 
may  be  seen  in  the  following  equation: — 

/OK 

C2H5I     +     S0<  ==     CaH6.S02.OK     +     KI  . 

M)K 

Ethyl  iodide.        Potassium  sulphite.        Potassium  ethylsulphite. 

These  compounds  were  supposed  to  be  identical  with 
the  sulpho  acids.  Indeed,  in  a  few  cases,  it  seems  that 
the  compounds  obtained  by  this  latter  method  must  have 
the  constitution  above  accepted  for  true  sulpho  acids. 
Nevertheless,  it  has  recently  been  shown  that  the  com- 
pounds obtained  from  the  sulphites  differ  from  true 
sulpho  acids;  that  the  former  are  really  ethers  of  sul- 
phurous acid,  and  not  substitution  products,  in  the  sense 
in  which  we  have  here  used  that  expression.  The  follow- 
ing experiments  furnish  the  proofs  : — 


156  CHEMICAL    COMPOUNDS. 

When  benzylchloride,  CfH5.CH.2Cl,  is  treated  with 
potassium  sulphite,  reaction  ensues  in  the  manner  above 
indicated,  viz.: — 

C6H5.CH2C1  +•  K.2S03  =  C6H5.CH2.S03K  -f  KC1. 

When  the  new  salt  is  treated  with  phosphorus  chloride, 
the  products  of  the  reaction  are  phosphorus  oxychloride, 
POC1S,  thionylchloride,  SOClz,  and  benzylcliloride, 
C6H5.CH,C1.  If  the  salt  had  been  derived  from  a 
true  snlpho  acid,  a  sulpho  chloride  of  the  formula 
CBHft.CH,.S02Cl  would  have  been  obtained.  It  is  hence 
concluded  that,  in  the  salt,  the  sulphur  does  not  hold  the 
rest  of  the  group  in  combination  with  the  h3Tdrocarbon 
residue,  but  that  this  is  accomplished  by  means  of  an 
oxygen  atom — the  group  having  the  constitution  ex- 
pressed by  the  formula 

— 0— S— 0— O— H;    or  by  0— 0— S— 0— H. 

The  formation  of  the  above  products  can  then  readily  be 
explained : — 

C6H6.CH2.O.S.O— OK     yields 


C6H5.CH,C1     Cl— S— 0— Cl 


C1K. 


The  oxygen  abstracted  will,  of  course,  form  phosphorus 
ox37chloride.  If  sulphur  had  occupied  either  of  the  posi- 
tions occupied  by  the  oxygen  atoms  in  the  above  formula 
which  are  abstracted,  one  of  the  products  of  the  reaction 
would  have  been  phosphorus  sulphochloride,  PSClo, 
whereas  not  a  trace  of  this  substance  could  be  detected. 

This  subject  requires  more  investigation  before  the 
conclusions  above  drawn  can  be  looked  upon  as  definitely 
settled.  In  all  probability  it  will  be  found  that  there  are, 
as  stated,  two  isomeric  groups,  S03H,  only  one  of  which 
can  give  true  sulpho  acids.  The  formula  above  given  for 
the  sulpho-acid  group  will  also  probably  be  found  to  be 
the  correct  one.  Further,  from  experiments  thus  far 
made,  it  seems  more  than  likely  that  these  groups  easily 
undergo  transformation,  their  atoms  being  rearranged. 

Constitution  of  the  ^Group  NO., — When  concentrated 
nitric  acid  is  allowed  to  act  upon  h}rdrocarbons,  etc., 


GENERAL    CONSIDERATIONS.  157 

hydrogen   is   frequently   replaced    by   the  group    N02, 
thus:  — 


C6H6       +       N0.2OH       =       CeH.-NO,       -f       H,O.. 

Benzene.  Nitric  acid.  Nitrobenzene. 

The  reaction,  as  will  be  noticed,  is  similar  to  that  which 
takes  place  when  sulphuric  instead  of  nitric  acid  is  used. 
Just  as  in  the  former  case  we  can  assume  that  the  group 
S03H  is  a  residue  of  sulphuric  acid,  so  in  the  latter  case 
•we  can  assume  that  the  group  NO2  is  a  residue  of  nitric 
acid.  The  formula  which  we  accept  for  nitric  acid  will 
show  us  the  constitution  of  the  group  N02.  Again, 
nitro  compounds  are  formed  by  treating  a  chloride, 
bromide,  or  iodide  of  a  hydrocarbon  residue  with  silver 
nitrite,  AgNOa,  the  reaction  taking  place  as  follows:  — 

CaH5I     +     AgN02    =     C2H5(N02)     +     AgL 

Ethyl  iodide.  Nitroethane. 

It  would  appear  from  the  latter  reaction  that  the  group 
N02  has  the  same  constitution  in  the  nitro  derivatives 
that  it  has  in  nitrons  acid  ;  but  this  is  in  reality  not  the 
case,  or,  at  least,  certain  facts  seem  to  indicate  clearly  a 
dissimilarity  of  the  groups. 

There  are  two  series  of  compounds  of  the  same  com- 
position, but  of  different  constitution,  both  of  which  con- 
tain the  group  N02.  The  members  of  one  of  these  series 
are  ethers  of  nitrous  acid.  If  nitrous  acid  contains 
hydroxyl,  then  the  ethers  would  have  the  general  con- 
stitution R  —  0  —  NO,  in  which  R  represents  a  hydro- 
carbon residue.  The  characterizing  feature  in  the  con- 
stitution of  these  ethers  is  the  same  as  that  which  we 
find  in  all  ethers,  viz.,  the  acid  group  is  combined  with 
the  hydrocarbon  residue  by  means  of  an  atom  of  oxygen. 
That  this  is  true  of  the  ethers  of  nitrous  acid  is  shown  by 
the  fact  that,  when  nascent  hydrogen  acts  upon  them, 
they  yield  the  alcohols  corresponding  to  the  hydrocarbon 
residues  which  they  contain,  and  at  the  same  time 
ammonia.  If  the  nitrogen  atom  had  been  directly  united 
with  the  hydrocarbon  residue,  we  would  have  found  it  in 
combination  with  this  residue  after  the  above  reduction. 
The  decomposition  which  actually  takes  place  may  be 
represented  thus:— 
14 


158  CHEMICAL    COMPOUNDS. 

Ether,     R— O— N=0     yields 

TT 

R_0— H     and     H— N/"      . 
\H 

Alcohol.  Ammonia. 

With   the   constitution   assumed  for   the   ether,  it  is. 
evident  that  the  formation  of  alcohol  by  the  addition  of 
hydrogen  would  necessitate  the  splitting  off  of  the  group 
containing  nitrogen. 

On  the  other  hand,  the  second  series  of  compounds  are 
not  ethers  of  any  acid,  but  are  true  substitution  products. 
They  consist  of  a  hydrocarbon  residue  combined  with  the 
group  N02  by  means  of  the  nitrogen  atom.  Their  general 
constitution  is  expressed  by  the  formula  R — N0a. 

This  conclusion  is  reached  by  considering  the  products 
of  the  reduction  of  nitro  compounds.  When  treated  with 
nascent  hydrogen,  they  yield  products  known  as  amine 
bases,  which  are  ammonia  in  which  one  hydrogen  atom 
has  been  replaced  by  a  hydrocarbon  residue.  The 
decomposition  is  represented  as  follows  : — 

R— NO,      +      6H         =         R— NH2      -f     2H20. 

Nitro  products.  Aminebase. 

In  the  product  obtained  in  this  case,  it  is  evident  that 
the  nitrogen  atom  is  in  direct  combination  with  the  hydro- 
carbon residue,  and  hence  we  can  assume  that  this  kind  of 
combination  also  existed  in  the  original  nitro  compound. 

Accepting  the  above  formula  for  nitro  compounds,  it 
is  difficult  to  see  how  they  can  be  formed  by  reaction 
with  silver  nitrite.  For,  if  the  hydrocarbon  residue  took 
the  place  occupied  by  the  silver  in  the  salt,  it  is  plain 
that  the  product  would  be  an  ether  which,  according  to 
what  has  already  been  said,  must  have  the  formula 
R — 0 — NO.  The  product,  however,  is  not  an  ether. 
Consequently,  some  other  change  besides  that  of  an  inter- 
change of  places  by  the  silver  atom  and  the  hydrocarbon 
residue  must  be  accomplished  at  the  same  time.  Conse- 
quent! j7,  further,  the  group  N02  in  nitrous  acid  has  a  con- 
stitution differing  from  that  of  the  group  N02  of  nitro 
compounds. 

As  to  the  respective' constitutions  of  these  two  groups 
we  see  that  in  nitrous  acid  we  have  in  all  probability  one 
hydroxyl.  This  gives  us  one  oxygen  atom  combined 


GENERAL    CONSIDERATIONS.  159 

by  one  affinity  with  hydrogen,  and,  on  the  other  side, 
with  nitrogen,  thus  :  H  —  0  —  N  —  .  The  only  thing  that 
is  otherwise  present  in  the  molecule  is  an  atom  of  oxygen 
which,  it  is  safe  to  suppose,  is  combined  by  both  its 
affinities  with  nitrogen  ;  whence  we  have  the  group 
—  0  —  N=0  as  the  characteristic  group  of  the  acid  and 
its  derivatives.  But  the  group  of  nitro  compounds 
unites  with  residues  by  means  of  its  nitrogen  atom,  as 
we  have  seen.  Hence,  we  can  conceive  of  two  formulas 

/o 

for  the  group  NO  ,  viz.,     —  N/        ,     in  which  the  nitro- 

NO 

^o 

£en  is  trivalent,  and     —  N<T        ,      in  which  the  nitrogen 

^O 

is  quinquivalent.  It  has  not  been  found  possible  to 
decide  which  of  these  two  formulas  is  the  correct  one. 

/° 

The  former,     —  NY    |     ,      is  now  more   commonly  ac- 


— NY    | 


cepted  than  the  latter.  The  fact  that  the  group  is  so 
readily  converted  into  the  group  NH2,  in  which  the 
nitrogen  atom  is  undoubtedly  trivalent,  makes  this  for- 
mula appear  probable. 

Constitution  of  the  Group  NO.  —  A  class  of  bodies 
which  have  recently  been  quite  fully  investigated  contain 
the  group  NO  in  the  place  of  hydrogen.  They  are  known 
as  nitroso  compounds.  Their  preparation  is  accomplished 
by  allowing  the  compound  NOBr  to  act  upon  mercury 
derivatives  of  hydrocarbons,  thus  :  — 

Hg(C.H6),    -f   NOBr  Hg(C6H5)Br  +   C6H5(NO). 

Assuming  that  nitrogen  here  acts  as  a  trivalent  ele- 
ment, and  oxygen  as  bivalent,  the  constitution  of  the 
group  NO  can  only  be  represented  by  the  formula  —  N=0, 
according  to  which  the  combination  between  the  hydro- 
carbon residue  and  the  substituting  group  in  nitroso 
compounds  is  accomplished  by  means  of  a  nitrogen  atom. 
This  view,  further,  is  in  accordance  with  the  fact  that,  by 
reduction,  the  group  NO,  like  NO.,  is  converted  into 
NH, 


160  CHEMICAL    COMPOUNDS. 

Constitution  of  the  Group  NH..  —  Compounds  which 
contain  the  group  NIia  are  called  amido  compound*. 
The  group  is  plainly  a  residue  of  ammonia  and  is  uni- 


valent,  having  the  constitution       —  N<^        .  These 

\H 

compounds  are  readily  obtained  by  the  action  of  nascent 
hydrogen  on  nitro  derivatives,  the  group  NO3  being 
hereby  converted  into  NH2.  Amido  substitution  products 
have  properties  which  ally  them  to  ammonia,  which  fact 
is  a  further  evidence  of  a  similarity  in  the  constitutions 
of  the  two.  They  have  basic  properties  in  the  same 
sense  that  ammonia  has  basic  properties,  i.  P.,  they  unite 
directly  with  all  acids  forming  salts.  Jn  addition  to  the 
above  method  of  formation,  we  have  also  the  action  of 
aqueous  ammonia  upon  chlorides,  bromides,  or  iodides  of 
h}rdrocarbon  residues. 


C2H.Br      -j-     NH3  C.2H5(NHJ     +     HBr. 

Ethyl  bromide.  Ethylamine. 

This  latter  method  indicates  very  decidedly  the  inti- 
mate connection  between  amido  compounds  and  ammonia. 

Constitution  of  the  Group  NH.— Finally,  there  are  a 
few  representatives  known  of  a  class  of  bodies  called 
imido  compounds.  These  contain  the  group  NH,  which 
is  bivalent,  and  hence  occupies  the  place  of  two  hydrogen 
atoms.  Like  amido  compounds,  they  may  be  considered 
as  derived  from  ammonia  by  the  replacement  of  two 
hydrogen  atoms  by  hydrocarbon  residues.  The  constitu- 
tion of  the  group  and  of  the  compounds  is*  readily  under- 
stood. We  have — 

C2H5Br)  C,H5) 

I     4-     NH3     =     C  H5  C  N      +      2HBr  . 
C2H5Br )  H       \ 

2  mol.  Ethyl  Diethylamine. 

bromide. 


The  various  classes  .of  chemical  compounds  which  we 
have  thus  studied  are  the  principal  classes  with  which  we 
have  to  deal.  There  are  a  few  other  classes,  among 
which  may  be  mentioned  the  so-called  mustard  oils  and 


GENERAL    CONSIDERATIONS.  161 

the  quinones.  These  will  be  considered  later,  in  connec- 
tion with  those  better  known  compounds  with  which 
they  are  most  closely  allied. 

In  classifying  compounds  we  have  distinguished  between 
general  classes  and  their  substitution  products.  This 
distinction  is  generally  justified,  though,  in  a  certain 
sense,  even  those  compounds  which  belong  to  the  general 
classes  are  substitution  products,  or,  at  least,  may  be 
considered  as  such.  This  is  particularly  the  case  witli 
the  compounds  of  carbon,  all  of  which  may  be  looked 
upon  as  derived  from  certain  hydrocarbons  by  the  re- 
placement of  something  else.  Thus  the  alcohols  are 
derived  from  hydrocarbons  by  replacing  hydrogen  by 
hydroxyl,  OH ;  the  acids  by  replacing  hydrogen  by  carb- 
oxyl,  CO. OH,  etc.  Speaking,  then,  of  all  the  groups 
which  have  been  studied  as  substituting  groups,  the  gene- 
ral statement  may  be  made  that:  In  all  substituting 
groups  the  characterizing  element  or  elements  may  be 
replaced  by  another  or  others  of  the  same  general  cha- 
racter. Thus,  as  we  have  already  seen,  0  may  be  replaced 
by  S.  Further,  S  may,  in  some  cases,  be  replaced  by  Se 
or  Te ;  N  may  be  replaced  by  P,  etc.  Thus  we  get  new 
compounds,  but  the  constitution  of  these  new  compounds 
is  the  same  as  that  of  those  from  which  they  are  derived, 
and  hence  they  require  no  separate  study  in  this  place. 


14* 


II. 

SPECIAL  STUDY  OF  THE   CONSTITUTION  OF 
CHEMICAL  COMPOUNDS. 

From  the  general  considerations  of  the  preceding  sec- 
tion, the  constitution  of  the  majority  of  compounds  will 
be  directly  understood.  In  the  following  section,  the 
constitution  of  individual  compounds  will  be  considered, 
in  so  far  as  any  special  consideration  of  them  is  neces- 
sary. Here  the  compounds  of  carbon  again  will  require 
most  of  our  attention,  because  more  is  known  concerning 
their  constitution  than  is  known  concerning  other  com- 
pounds. It  will  be  found  that  the  same  atoms  may 
frequently  be  arranged  in  different  ways,  giving  rise  to 
different  compounds. 

Isomerism.  —  There  are  two  ways  in  which  bodies  may 
contain  the  same  elements  in  the  same  proportions  by 
weight,  and  still  be  different  bodies  :  — 

1.  The  atoms  or  groups  composing  the  body  may  be 
arranged  different^  in  the  molecule,  as  above  stated. 
Thus  we  have  ammonium  cyanate,  CN(ONH4),  and 

XNH2 
urea  or  carbamide,      C0<  .         These  bodies  con- 


tain  the  same  atoms  in  the  same  number  in  their  mole- 
cules.    Such  bodies  are  called  metameric. 

2.  Bodies  may  have  the  same  composition,  but  have 
different  molecular  weights.  Thus  we  have  acetylene, 
C,H?,  and  benzene,  C6H6.  Such  bodies  are  called  poly- 
meric. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        163 


COMPOUNDS  NOT  CONTAINING  CARBON. 

The  compounds  which  the  monovalent  elements,  hydro- 
gen, chlorine,  bromine,  iodine,  fluorine,  sodium,  potassium, 
lithium,  caesium,  rubidium,  silver,  form  with  each  other 
have  the  simplest  constitution  of  which  we  have  any  con- 
ception. They  require  no  study  here. 

Compounds  of  Chlorine,  etc.,  with  Oxygen,  and  Oxygen 
and  Hydrogen  —  Hydrogen  peroxide  has  the  empirical 
formula  H,2O2.  If  in  this  compound  the  oxygen  is  bivalent, 
the  arrangement  of  the  atoms  can  be  most  readily  imagined 
to  be  in  accordance  with  the  formula  H — O — 0 — II. 
There  is,  indeed,  no  independent  proof  of  the  correctness 
of  this  formula,  but  no  other  formula  expresses  the  con- 
stitution at  all  satisfactorily,  and  hence  this  is  accepted 
as  correct. 

The  acids  which  chlorine  forms  with  oxygen  and 
hydrogen  are  most  frequently  considered  as  represented 
by  the  following  formulas  : — 

Cl — 0 — H,  hypochlorous  acid, 

Cl— 0— 0— H,  chlorous  acid, 

Cl— 0— 0— 0— H,  chloric  acid, 

Cl— 0— O— 0— 0— H,        perchloric  acid  ; 

and  the  compounds  of  chlorine  with  ox}Tgen  alone  by  the 

following  formulas: — 

Cl — 0 — Cl,  hypochlorous  anhydride, 

Cl — 0 — 0— O — Cl,  chlorous  anhydride, 

Cl — 0 — 0 — 0 — 0 — Cl,         chlorous-chloric  anhydride. 

The  corresponding  compounds  of  bromine,  iodine,  and 
fluorine,  as  far  as  they  are  known,  are  represented  by 
corresponding  formulas.  It  must  be  confessed  that  these 
formulas  are  by  no  means  well  established,  and  it  is  very 
possible  that,  when  the  compounds  themselves  shall  have 
been  studied  more  carefully,  they  will  be  found  to  be 
incorrect. 

Compounds  of  Sulphur,  etc.,  with  Oxygen,  and  Oxygen 
and  Hydrogen. — Sulphur  forms  a  number  of  compounds 
with  oxygen,  and  with  oxygen  and  hydrogen,  some  of 
which  have  been  carefully  studied.  The  compounds  with 


164  CHEMICAL    COMPOUNDS. 

oxygen  alone  are  sulphur  dioxide,  SO2,  and  sulphur  tri- 
oxicle,  SO^.  If  in  these  compounds  both  the  sulphur  and 
the  ox37gen  are  bivalent,  we  have  the  following  possible 
formulas : — 

For  SO,,    S/  I  ,     S/'         ,     — S— 0— 0—  ; 

X)          \0— 

/0\  ,0-0- 

ForSO,,    S<       >o,    S<  ,    — S— O— 0— 0— . 

X0X  X0— 

/o 

As    is   plain,   all  the   formulas   except        S<^  |         and 

/0\ 
S<f       \O       represent   the   compounds   as    un  saturated. 

XX 

Now  S02  conducts  itself  like  an  unsaturated  compound  ; 
it  combines  directly  with  chlorine,  forming  the  product 
SO2C12,  and  with  oxygen,  forming  S03,  so  that  it  seems 

/O- 
possible  that  the  formula  S<^  or  — S — 0 — 0 —  may 


in  reality  express  its  constitution.     Again,  some  chemists 
consider  sulphur  quadrivalent  or  sexivalent  in  these  com- 


pounds,  which  would  give  the  formulas       S/  and 


/-o 

S—  0  .      Perhaps  the  most  commonly  accepted  formulas 

\0 

/°  /o\ 

are    S<  and     S<        >0  ,     though   the   reasons  for 


accepting  them  are  not  very  good  ones. 
The  acids  of  sulphur  are  the  following:  — 
H.2SO3,      sulphurous  acid. 
H2S04,      sulphuric  acid. 
HaS207,     pyrosulphuric  acid. 
HaS2O3,     hyposulphurous  or  thiosulphuric  acid. 
H2S3O6,     hyposulphuric  or  dithionic  acid. 
H8S306,     trithionic  acid. 
H2S4O6,     tetrathionic  acid. 
H2S506,     pentathionic  acid. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        165 

Sulphurous  Acid,  H.2S0.6. — Only  derivatives  of  this 
acid  are  known — as  the  salts  and  ethers.  From  a  study 
of  these  derivatives,  conclusions  have  been  drawn  con- 
cerning the  constitution  of  the  acid  itself.  The  consti- 
tution of  sulphurous  acid  is  expressed  by  very  different 
formulas  by  those  who  consider- sulphur  as  bivalent,  and 
those  who  consider  that  it  may  and  does  'act  as  quadri- 
valent or  sexivalent.  Up  to  the  present,  no  conclusive 
evidence  of  either  view  has  been  furnished.  We  propose 
to  consider  sulphur  as  bivalent,  and  to  show  to  what 
conclusions  we  shall  then  be  led.  This  we  can  do  without 
interfering  with  our  main  object,  which  is  to  show  more 
particularly  the  methods  of  thought  and  experiment 
that  lie  at  the  foundation  of  constitutional  formulas  in 
general. 

If  then  sulphur  is  bivalent,  we  have  as  probable  for- 

0— OH  0— OH 

mulas  for  sulphurous  acid   S<f  or  O<f 

X)H  XSH 

For  some  time  the  latter  formula  was  accepted,  in  con- 
sequence of  the  fact  that  the  sulpho  acids  are  so  readily 
formed  from  salts  of  sulphurous  acid.  We  have  seen 
that,  in  the  sulpho  acids,  the  group  S03H  is  combined 
with  hydrocarbon  residues  by  means  of  the  sulphur  atom 
Now,  as  the  group  S03H  can  apparently  be  obtained 
from  sulphites,  such  a  constitution  was  assigned  to  the 
latter  as  to  make  the  connection  between  them  and  the 
sulpho  acids  clear.  This  can  best  be  accomplished  if  we 

,0— OM 
accept    the   formula     O^  for     the     sulphites. 

NSM 

If  the  metal  which  is  in  combination  with  sulphur  in  this 
formula  be  replaced  by  a  hydrocarbon  residue,  the  latter 
would  be  in  combination  with  sulphur  also.  On  the 
other  hand,  if  we  accept  the  former  of  the  two  formulas 

,0— OH 
above  proposed,  viz.,     S<;  ,     it     would    appear 

M)H 

difficult  to  obtain  sulpho  acids  from  sulphites  by  a  simple 
interchange  of  atoms  or  groups.  Recent  experiments 
seem  to  show  that  the  compounds  obtained  from 


166  CHEMICAL    COMPOUNDS. 

the  sulphites,  which  were  formerly  looked  upon  as 
sulplio  acids,  are  in  reality  ethers  of  sulphurous  acid, 
and  not  sulpho  acids;  that  in  them,  further,  the  hydro- 
carbon residue  is  not  in  combination  with  sulphur  (see 
ante,  p.  155).  From  this  it  is  concluded  that  the  formula 

/O—  OH 
of  sulphurous   acid  is     S\ 

M)H 

Sulphuric  Acid,  H.,S04.  —  The  same  reasons  which  lead 
us  to  accept  the  formula  —  S  —  0  —  0  —  OH  for  the  charac- 
terizing group  of  sulpho  acids  (see  ante,  p.  156),  also  lead 
us  to  accept  this  formula  as  part  of  the  whole  formula  of 
sulphuric  acid  But  we  know  that  sulphuric  acid  con- 
tains two  hydroxyl  groups,  consequently,  adding  one 
hydroxyl  group  to  the  above  group,  we  have  as  the  for- 
mula for  sulphuric  acid,  HO—  S—  0—0—  OH. 

Another  reason  for  accepting  this  formula  is  found  in 
the  following  facts:  By  the  action  of  the  chloride  of 
phosphorus  on  sulphuric  acid,  sulphuryl  oxychloride, 


S02<(          ,  is  obtained.     In  this  compound  the  chlorine 

\C1 

in  all  probability  occupies  the  place  which,  in  sulphuric 
acid,  is  occupied  by  hydroxyl.  This  appears  still  more 
probable  when  we  consider  that  the  oxychloride  is  con- 
verted into  the  acid  by  simple  treatment  with  water. 
Now,  if  sulphuryl  oxychloride  is  treated  with  ethylene,  a 

'  /OC2H5 
compound,     S0,<\  ,      is  formed  by  direct  addition. 

\G! 
Three  formulas  are  possible  for  this  compound,  viz.:  — 

C2H.—  0—  O—  S—  0—  Cl  ; 
C2H  —  O—  S—  0—  O—  Cl  ; 
Cl—  S—  O—  O—  O—  C,H5. 

Further,  if  sulphuryl  oxychloride  is  treated  with  alcohol 


it  yields  ethylsulphuric  acid,    SO.  .    If  the  for- 

\OC2H5 
mula  of  sulphuric  acid  were  symmetrical  — 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        16t 

HO— 0— S— 0— OH,  then  ethylsulphuric  acid  would  be 
C2H..O — 0 — S— 0 — OH,  and  a  chloride  derived  from  it 
would  be  C2H5.0— 0— S— 0— Cl.  Sulphuryl  oxychlor- 
ide  would  have  the  formula  HO — 0 — S — 0 — Cl,  and  the 
compound  derived  from  it  by  the  addition  of  ethylene 
would  be  C,H,.O— 0— S— 0— Cl ;  or  this  latter  would 
be  identical  with  the  chloride  of  ethylsulphuric  acid. 
The  two  are,  however,  not  identical,  and  hence  it  follows 
that  the  formula  of  sulphuric  acid  cannot  be  symmetrical, 
thus,  HO — O — S — O — OH.  If  it  is  not  symmetrical, 
however,  it  must  be  HO— S— 0— O— OH. 

Pyrosulphuric  Acid,  H2S.20:. — Pyrosulphuric  acid  is  a 
partial  anhydride  of  sulphuric  acid  obtained  by  abstract- 
ing one  molecule  of  water  from  two  molecules  of  the 
acid : — 

OH 

SO/  I  /OH 


OH  SO. 


,OH 


SO 


SO 


J 

2  mol.  Sulphuric  acid.  Pyrosulphuric  acid. 

The  general  statements  made  in  connection  with  the 
subject  of  anhydrides  contain  the  proofs  for  the  above 
formula.  Of  course,  the  groups  SO2  contained  in  this 
anhydride  are  supposed  to  have  the  same  constitution 
that  they  have  in  sulphuric  acid. 


Hyposulphurous  or  Thiosulphuric  Acid, 
This  acid  is  usually  considered  as  sulphuric  acid,  in  which 
one  of  the  hydroxyl  groups  has  been  replaced  by  the 
sulphuryl  group  SH,  thus:  — 

OH  ,SH 


so  so 


M)H 

Sulphuric  acid.  Thiosulphuric  acid. 

This  formula  is  rendered  probable  by  the  fact  that 
thiosulphuric  acid  may  be  obtained  from  sulphuric  acid 
by  treating  the  latter  with  phosphorus  sulphide.  The 
action  of  the  latter  reagent  consists  in  replacing  oxygen 


168  CHEMICAL    COMPOUNDS. 

by  sulphur.  And  the  oxygen  of  the  hydroxyl  group  is, 
in  general,  more  susceptible  to  the  influence  of  reagents 
than  that  contained  in  the  group  SO.,. 

Further,  thiosulphuric  acid  is  obtained  by  allowing 
hydrogen  sulphide  to  act  upon  sulphur  trioxide,  just  as 
sulphuric  acid  is  obtained  by  allowing  water  to  act  upon 
sulphur  trioxide. 

/OH 

SO.,.0       +       H20        =        SO 


/SH 
SO..O       +      H.,S  S02 


Other  Polythionic  Acids.  —  Dithionic  acid,  H2S/)6,  is 
considered  as  related  to  pyrosulphuric  acid,  thus:— 


SO  SO 

•      so/0     ;  ./-so0 

\OH 

Pyrosulphuric  acid.  Ditbionic  acid. 

Trithionic  Acid,  H.^S^O^,  ma}^  be  considered  as  derived 
from  pyrosulphuric  acid  in  the  same  way  that  thiosulphuric 
acid  is  derived  from  sulphuric  acid,  thus  :  — 

,OH  SH 


so,, 

OH 

Pyrosulphuric  a"id.  Trithionic  acid. 

Or,  it  may  be  that  trithionic  acid  is  an  anhydride  of 
thiosulphuric  acid: — 

OH     1 
SO/  /OH 

\5H  SO/ 

-  H2S        =  >S       . 


SH 


SO., 


so. 


OH 


\OH 

2  mol.  Thiosulphuric  acid.  Tritbioaic  acid. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        169 

The  latter  view  is  rendered  probable  by  the  fact  that 
trithionates  are  formed  when  double  salts  of  thiosul- 
phuric  acid  are  boiled  with  water.  The  reaction  takes 
place  according  to  the  following  equation  : — 

2AgKS80,  Ag.2S     +     K2S806- 

/OK  1 

S02<  /OK 

\3Ag  I  S02< 


SO 


-SAg 


OK 


J 

When  potassium  trithionate  is  boiled  with  potassium 
sulphide,  potassium  thiosulphate  is  formed — 

K.2S306       +       K2S  2KaSA- 

This  reaction  also  indicates  an  intimate  relation  between 
thiosulphuric  and  trithionic  acids. 

Tetrathionic  Acid,  1T.2S406,  is  obtained  b}'  the  action  of 
iodine  on  sodium  thiosulphate,  thus: — 

NaS0 


2  mol.  Sodiuin  Sodium  tefrathionate. 

tl)iosulohat( 


/ONa 
SO/  S02<_ 


,SNa 


2NaI. 


so. 


SO. 


ONaj  xONa 


Hence,  the  formula  of  the  acid  from  which  the  latter 
salt  is  derived  is  accepted  for  tetrathionic  acid. 

Pentathionic  Acid,  H2S5O(.,  is  obtained  by  treating 
barium  thiosulphate  with  sulphur  bichloride  S2C12.  Tlie 
latter  compound  has  the  constitution  Cl — S — S — Ci ; 
consequently,  the  reaction  may  be  interpreted  most 
readily  as  follows: — 
"  15 


170  CHEMICAL    COMPOUNDS. 

/OH   1  /OH 

S02<  SO/ 

'\3H  N3-S 

V  +  01—  S—  S-C1  =  I    +  2HC1  . 

XSH    I  /S—  S 

SQ,  S0 


2  mol.  Thiosul-  Pentatbionic  acid. 

phuric  acid. 


The  corresponding  acids  of  selenium  and  tellurium, 
as  far  as  the}'  are  knowji,  are  represented  by  similar 
formulas. 

Compounds  of  Nitrogen  with  Oxygen,  and  with  Oxygen 
and  Hydrogen.  —  Nitrogen  forms  with  oxygen  the  follow- 
ing compounds:— 

N20,  nitrons  oxide  or  nitrogen  monoxide. 

NO  or  N202,  nitrogen  dioxide. 

N808,  nitrogen  trioxide. 

N02  or  N204,  nitrogen  tetroxide. 

N20S,  nitrogen  pentoxide. 

The   constitution  of  nitrogen  monoxide   is  usually  ex- 


pressed  by  the  formula 

O 

Nitrogen  dioxide  is  —  N=0,  and  is,  therefore,  unsatu- 
rated.  The  readiness  with  which  it  combines  with  oxygen 
and  chlorine  indicates  its  unsaturated  condition. 

Nitrogen  tetroxide  is  —  0  —  N=O,  and  is  also  unsatu- 
rated. This  formula  is  given  the  compound  because  it  is 
obtained  so  easily  from  the  monoxide,  and  is  converted 
into  the  latter  so  easily. 

Nitrogen  trioxide  is  the  anhydride  of  nitrous  acid. 
Hence  its  formula  depends  upon  that  of  nitrous  acid 
(which  see). 

Nitrogen  pentoxide  is  the  anhydride  of  nitric  acid. 
Its  formula  depends,  upon  that  of  nitric  acid  (which  see). 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        17i« 

There  are  two  acids  of  nitrogen,  viz.: — 

HNO.^,         nitrous  acid. 
HN03,         nitric  acid. 

Nitrous  Acid,  HNO,. — The  proofs  for  the  presence  of 
the  group  — 0 — N— 0  in  nitrous  acid  have  been  given 
under  the  head  of  nitro  compounds  (which  see).  The 
acid  has  the  constitution  H — 0 — N=0.  From  this 
formula  we  derive  that  of  the  anhydride,  nitrogen  tri- 
oxide,  thus : — 

I    —    HO    =  \0  . 

0=N— 0— H  )  0=N/ 

Two  mol.  Nitrous  acid.  Nitrogen  trioxide. 

Nitric  Acid,  HN03 — It  has  been  shown  above  (see 
Nitro  Compounds)  that  the  group  contained  in  nitro 

/O 
compounds    has    probably   the    constitution     — N<^    |  . 

XO 

It  being  further  probable  that  this  group  is  contained 
in  nitric  acid,  the  constitution  of  the  latter  would  be 

H — 0 — N<^  |  .     The  fact,  that  nitric  acid  is  so  volatile, 

X0 

seems  to  indicate  that  the  nitrogen  contained  in  it  is 
trivnlent  rather  than  quinquivalent.  We  saw  above 
other  facts  indicating  the  same  thing,  so  that  now  the 
above  formula  is  pretty  generally  accepted. 

Hydroxylamine,  H.^NO. — This  substance  is  a  strong 
base  conducting  itself  like  ammonia.  By  appropriate 
reactions  it  can  be  shown  that  two  of  the  hydrogen  atoms 
contained  in  the  compound  differ  from  the  other  one. 
These  facts  prove  that  hydroxylamine  is  a  derivative  of 

/H 

ammonia,  and  the  formula  is  consequently     N — H 

\O— H 


172  CHEMICAL    COMPOUNDS. 

Compounds  of  Phosphorus  with  Oxygen,  and  with 
Oxygen  and  Hydrogen. — Phosphorus  forms  two  oxides, 
viz.: — 

P-jOy,  phosphorus  trioxide. 

^•fiv  phosphorus  pentoxide. 

The  former  is  usually  considered  as  derived  from  trivalent 
phosphorus,  and  to  it  is  consequently  given  the  formula 

/0\ 

P— 0— P  . 

\o/ 

The  latter,  however,  is  considered  as  derived  from 
quinquivalent  phosphorus,  and  to  it  is  given  the  formula 

/-°-\ 

l/0\| 
P— 0— P    . 

l\0/l 
\-0-/ 

These  formulas  are  purely  hypothetical. 
There  are  several  acids  of  phosphorus,  viz.: — 

H3PO2,  hypophosphorous  acid. 

H3POS,  phosphorous  acid. 

H3P04,  phosphoric  acid. 

H4P2O7,  pyrophosphoric  acid. 

HPO,,  metaphosphoric  acid. 


Hypophosphorous  acid,  H3POV,  is  monobasic,  and  hence 
onty  one  hydroxyl  group  is  assumed  as  present  in  its  mole- 

H 

cule.    This  gives  the  formula    H2PO.OH    or     P/'        , 

I      O 

OH 
in  which  the  phosphorus  atom  is  quinquivalent. 

Phosphorous  Acid,  H,P03. — In  regard  to  the  constitu- 
tion of  this  acid,  two  views  "are  held.     According  to  the 

/OH 
first,  the  formula  of  the  acid  is     P — OH  ,     the     phos- 

\OH 


CONSTITUTION    OP    CHEMICAL    COMPOUNDS.        173 

phorus  being  trivalent.      According  to  the  second,  the 


formula    is      P^  ,     the    phosphorus    being    quin- 

|    \OH 
OH 

quivalent.  If  the  former  formula  is  correct,  the  acid 
ought  to  be  tribasic.  In  most  of  its  salts,  however,  it  is 
only  bibasic.  Still  ethers  are  known  which  are  evidently 
derived  from  a  tribasic  acid,  as,  for  instance,  P03(CaH.)3; 
and  it  has,  further,  recently  been  shown  that  a  salt  of  the 
acid  exists  in  which  there  are  three  atoms  of  a  monova- 
lent  metal  to  the  molecule.  These  latter  facts  would 
lead  to  the  formula  P(OH)3.  The  fact,  also,  that  phos- 
phorous acid  is  produced  by  simply  treating  phosphorus 
trichloride  with  water,  is  in  accordance  with  this  formula. 
We  have 


Cl  H 

Cl     +     H 
Cl  H 


HO  /OH 

HO     =     Pf  OH     +     3HC1 

HO  \OH 


On  the  other  hand,  the  following  facts  speak  for  the 
O 

formula    P<^  : — 

|    M)H 
OH 

When  benzene  is  treated  with  phosphorus  trichloride, 
under  appropriate  conditions,  the  following  reaction  takes 
place : — 

PC13     +     C6H6     =     PC12(C6H5)     +     HC1. 

When  the  main  product,  phosphenyl  chloride,  is  treated 
with  water,  the  chlorine  is  eliminated,  and  a  compound 
of  the  composition  P02H2(C6H5)  is  formed.  The  formula 
of  this  compound  may  be  either — 

^°H  I/*    ,;  .'•_'    ,: 


1,    POH     ;      or,      2, 

6H5  |      OH 

C6H5 

15* 


174  CHEMICAL    COMPOUNDS. 

If  the  latter  is  the  formula,  then  we  can  conclude  that 
the   constitution   of  phosphorous   acid   is   similar,   i.e., 


P\ 

|  XOH    * 

OH 

If  formula  1  is  correct,  then  by  the  action  of  phos- 
phorus pentachloride  upon  the  compound  the  following 
reaction  ought  to  take  place  :  — 

I.  P(OH)3C6H5  +  2PC15  =  PC12(C6H5)  + 

2(POC13)  +  2HC1. 

If,  however,  formula  2  is  correct,  then  under  the  same 
conditions  the  following  reaction  would  take  place:  — 

II.  OPH(OH)C6H5  +  2PC13  =  OPC12  (CfiH.)  -f 

POC1         PC1          2HC1. 


In  the  former  case,  phosphenyl  chloride,  PC12(C6H5), 
would  be  formed  ;  in  the  latter,  phosphenyl  oxychloride, 
POC12(C6H5).  Direct  experiments  showed  that  phos- 
phenyl oxychloride,  phosphorus  ox3'chloride,  and  phos- 
phorus trichloride  are  formed,  and  consequently  the 
formula  OPH  (OHjC6H5  is  correct;  and  phosphorous 

f  /H 

acid  by  analogy  is  OPH(OH)2   'or      P< 

I  NDH 

OH 


Phosphoric  Acid,  H3P04. — This  acid  is  tribasic,  and 
hence  three  hydroxyl  groups  are  assumed  to  be  present 
in  it.  From  this  follows  directly  the  formula  PO(OH)S. 
The  question  still  remains  whether  the  phosphorus  is 
quinquivalent  or  trivalent  in  the  acid.  In  the  former 


-OH 

case,  the  formula  would  be        P<(  ;      in  the  latter, 

|  >OH 
OH 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        175 

O— OH 


.     Phosphoric  acid  is  obtained  by  treating 
XOH 
phosphorus  oxychloride  with  water.    If  the  oxychloride  is 

O 

II /ci 

,      then  we  would    expect    phosphoric    acid    to 

Cl 

have  the  former  of  the  two  formulas  given.     If  the  oxy- 

/O— Cl 
chloride    is      P: — Cl          ,     then  the  acid  has  probably 

\C1 

the   latter   of  the   two   formulas   given.     The   formulas 

/OH  |f     Cl 

P<(  and          P<(  are    usually    accepted, 

|    M)H  I   \C1 

OH  Cl 

though  without  positive  proofs  of  their  correctness. 

Pyrophosphoric  Acid,  H4P^O. — This  is  a  partial  an- 
hydride of  phosphoric  acid  formed  by  abstracting  one 
molecule  of  water  from  two  molecules  of  the  acid,  thus: — 

/,O 


»^OH 
^OH 
\OH 

,OH 
/OH 


P< 


>O 
OH 
OH 


—     H.,0     =          \0 


P< 


OH 
OH 


\0 

2  mol.  Phosphoric  acid.  Pyrophosphoric  acid. 

The  constitution  is  readily  understood  by  the  aid  of 
the  general  remarks  on  the  subject  of  anhydrides. 


176  CHEMICAL    COMPOUNDS. 

Metaphosphoric  Acid,  HPO.A.  —  This  acid  has  a  compo- 
sition analogous  to  that  of  nitric  acid,  HNOa.  It  is,  like 
pyrophosphoric  acid,  a  partial  anhydride  of  phosphoric 
acid,  formed  by  abstracting  one  molecule  of  water  from 
one  molecule  of  the  acid,  thus  :  — 


H0  P-OH 


Phosphoric  acid.  Metaphosphoric  acid. 

Accepting  the  formula  for  phosphoric  acid  which  is 
employed  in  this  equation,  the  constitution  of  metaphos- 
phoric  acid  would  be  that  which  is  expressed.  Nitric 
acid,  it  will  be  remembered,  has  probably  the  constitution 

/O 

HO—  N          . 


The  relations  between  phosphoric  acid  and  its  anhy- 
drides are  shown  by  the  following  tables: — 

2(H3P04)  —  H,O  ==  H4P207,  pyrophosphoric  acid. 
2(H,P04)  —  2H2O  =  2HP03,  metaphosphoric  acid. 
2(H3POJ  —  3H20  =  P205,  phosphoric  anhydride. 


P205  -|~  H20  =  2HP03,  metaphosphoric  acid. 
P205  -f  2H20  =  H4P2O7,  pyrophosphoric  acid. 
P205  +  3H2°  =  2H3i>°4j  phosphoric  acid. 


Arsenic,  antimony,  and  bismuth  form  some  compounds 
analogous  to  those  of  phosphorus  here  described.  What 
has  been  said  of  the  constitution  of  the  latter  holds 
good  of  the  constitution  of  the  former. 

Compounds  of  Boron  with  Oxygen  and  with  Oxygen 
and  Hydrogen. — Boron  forms  only  one  oxide,  viz.,  B203, 
known  as  boron  trioxide.  The  acid  to  which  it  cor- 
responds is  B(OH)b.  When  boric  acid,  6(011)^,  is 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        177 

heated  for  some  time  at  100°  it  loses  a  molecule  of  water, 
a  partial  anhydride  being  formed,  thus:  — 

/OH  ^0 

B—  OH      —     H0  B—  OH    . 


When  this  anhydride  is  heated  to  a  much  higher  tem- 
perature, it  is  converted  into  B203,  thus  :  — 


B 


B 


OH 


,0 


B 


B/ 

^O 


The  formulas  of  these  anhydrides  here  given  are  purely 
hypothetical. 

Compounds  of  Silicon  with  Oxygen  and  with  Oxygen 
and  Hydrogen. — Silicon  bears  a  close  analogy  to  carbon 
in  some  respects.  It  usually  acts  as  a  quadrivalent  ele- 
ment, as  is  seen  in  the  compound  SC14.  When  this 
chloride  is  treated  with  water,  we  should  expect  as  a 
product  Si(OH)4,  which  may  be  considered  as  the  normal 
acid  of  silicon.  It  appears  to  be  possible  to  obtain  this 
acid,  but  it  is  very  unstable.  It  loses  water  easily,  and 
thus  yields  a  partial  anhydride,  SiOHH2,  thus: — 

Si(OH)4    —    H20     =    SiOaH2    =    SiO(OH)a. 

This  compound  is  usually  called  silicic  acid,  as  from  it 
are  derived  most  of  the  silicates. 

If  heated,  this  acid  yields  complicated  polysilicic  acids, 
which,  in  their  turn,  are  partial  anhydrides.  They  are 
formed  by  the  union  of  two  or  more  molecules  of  silicic 
acid  and  the  abstraction  of  varying  amounts  of  water 
from  them.  Examples  of  such  polysilicic  acids  are — 

Si203(OH)l2,  Si304(OH)4,  Si305(OH)2,  Si407(OH)2,  etc., 

some  of  which  are  found  in  nature,  as  opal,  hydrophane, 
etc.  As  final  product  of  the  action  of  heat  on  silicic 
acid,  we  have  silicon  dioxide,  Si02. 


178  CHEMICAL    COMPOUNDS. 

Salts.  —  The  constitution  of  the  most  important  acids 
being  thus  understood,  that  of  the  salts,  in  general, 
requires  no  special  consideration  ;  for  we  have  seen  that 
the  salts  are  very  simple  derivatives  of  the  acids.  There 
are  a  few  metals  and  groups,  however,  which  have  the 
property  of  yielding  peculiar  salts,  and  these  require  a 
brief  consideration. 

Ammonium  Salts.  —  When  ammonia,  NH3,  acts  upon 
any  acid,  a  salt  is  formed  by  direct  addition,  thus:  — 

NH3         +         HCl  =          (NHJC1 

Ammonium  chloride. 

NH3         +         HN03       =  (NH4)N03 

Ammonium  nitrate. 

2NH3         +         H2S04       =  (NH4\S04. 

Ammonium  sulphate. 

The  salts  thus  formed  are  similar  to  the  salts  of  potas- 
sium, sodium,  etc.,  KC1,  KN03  K;2SO4,  etc.  They  con- 
duct themselves  like  true  metallic  salts.  Hence,  the 
group  NH4,  which  is  contained  in  them,  is  supposed  to 
play  the  part  of  a  metal,  and  to  it  the  name  ammonium 
is  given.  Accordingly,  the  salts  are  called  ammonium 
salts.  These  have  been  referred  to  incidentally  under  the 
head  of  valence.  It  was  shown  that  in  them  the  nitrogen 
is  quinquivalent.  The  formulas  of  the  above  salts  are, 
accordingly  — 


—  H   ,  N—  H       ,  etc. 


Salts  of  Copper  and  Mercury. — Copper  and  mercury 
form  two  series  of  salts,  of  which  the  following  are 
examples: — 


Hg,Cla,        HgCl, 
Hga(NOs)a,  Hg(N08), 
Hg,(S04),   Hg(S04). 

If  we  determine  tlie  formulas  of  the  two  chlorides  of 
mercury  by  the  aid  of  the  specific  gravity  of  their  vapors, 


Cu2Cly,        CuCl, 

Cu2(N03X,Cu(NOs\ 

Cu,(S04), 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        179 

we  are  led  to  HgCl  and  HgCl.2.  According  to  these  for- 
mulas, mercury  is  bivalent  and  the  compound  HgCl  is 
unsaturated.  It  has  been  supposed,  however,  that  the 
formula  of  the  chloride  HgCl  in  the  solid  condition  is  in 

HgCl 
reality  Hg.CL,  and  that  it  has  the  constitution 

HgCl 

Hg 

The  group  is  bivalent  as  well  as  the  mercury  atom 

Hg 

itself,  and  thus  the  above  two  series  of  salts  are  explained. 
The  same  explanation  is  given  for  the  corresponding 
salts  of  copper, 

A  large  number  of  compounds  are  known  which  are 
derived  from  salts  of  ammonium  and  contain  copper  and 
mercury.  They  seem  to  consist  of  ammonium  salts,  in 
which  a  portion  of  the  hydrogen  of  the  ammonium  groups 
has  been  replaced  by  copper  or  mercury,  thus  :  — 


01    4_  9^H  s  He-  C1       Dimercury  diamido- 

--  »*«,  ==  -Ug3C1  ,  chloride. 


N03.NH2Hg^,  Dimercuryamine  nitrate. 

These  formulas  are  purely  hypothetical. 

Similar  compounds  are  formed  with  other  metals,  par- 
ticularly with  cobalt,  which  furnishes  a  very  large  number 
of  interesting  substances  of  this  kind.  These  are  too 
complicated  and  too  little  understood  to  permit  the  draw- 
ing of  positive  conclusions  concerning  their  constitution. 
Their  study  promises  important  results. 

Salts  of  Tron  and  Chromium.  —  Iron  and  chromium 
form  two  series  of  salts,  as  follows  :  — 


FeCla, 
FeiNOj., 
FeS04, 

Fed,, 
Fe(N03\, 
Fe.(SOA. 

or  Fe2Cl,. 

or  Fe.XN03)c. 

CrCl2, 
Cr(NO,),, 
CrS04, 

CrCl,, 
Cr  N08)s, 
Cr/S04)8. 

or  Cr.2Clfi. 
or  Cr.(N03)6. 

In  regard  to  the  formula  of  the  second  chloride  of  iron 
above  represented  by  FeCl^  or  FeaCl0,  there  is  still  doubt. 


180  CHEMICAL    COMPOUNDS. 

A  determination  of  the  specific  gravity  of  its  vapor  led 
to  the  formula  Fe^Cl,.,  whereas  a  determination  of  the 
specific  gravity  of  certain  other  derivatives  of  the  same 
constitution  as  the  chloride  led  to  the  simpler  formula. 
If  FeCl,  is  correct,  the  iron  atom  in  the  compounds  cor- 
responding to  this  chloride  is  trivalent,  whereas,  if  Fe2CL 
is  correct,  the  iron  atom  is  probably  quadrivalent,  and 

Cl     Cl 

the  constitution  of  the  compound  is     Cl — Fe — Fe — Cl . 

A  J. 

Of  course,  the  formulas  for  the  salts  of  iron  will  depend 
upon  those  which  we  accept  for  the  two  chlorides.  It  is 
most  commonly  considered  that  iron  may  be  bivalent  or 
quadrivalent. 


The  salts  of  manganese  resemble  those  of  iron  and 
chromium. 

Salts  of  Aluminium. — The  determinations  which  leave 
us  in  doubt  concerning  the  formula  of  the  higher  chloride 
of  iron  also  leave  us  in  doubt  concerning  the  formula  of 
the  chloride  of  aluminium.  It  is  either  A1C1{  or  Al2Clb, 
and  aluminium  is  either  trivalent  or  quadrivalent. 

Metal  Acids. — The  four  metals,  iron,  chromium,  man- 
ganese, and  aluminium,  form  hydroxides  of  the  general 
formula  MO. OH,  which  conduct  themselves  like  weak 
acids,  forming  salts  with  some  metals.  Thus,  we  have 
A10.0K  and  AlO.ONa,  salts  of  the  hydroxide  A10.0H.* 

Iron,  manganese,  and  chromium  yield  acids  of  the 
general  formula  M04H,.  Thus  we  have  FeO4H,,  MnO4H2, 
and  Cr04H2.  These  acids  are  analogous  to  sulphuric 
acid  H.,S04,  and  a  close  resemblance  is  noticed  between 
the  salts  of  sulphuric  acid  and  those  of  chromic  acid, 
which  is  the  best  known  of  the  three  above-named  acids. 

*  Of   course,    if  the*  chloride    is   A1.,C16,    this    compound    is 
,,  and  the  salts  have  a  corresponding  composition, 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.       181 

If  the  metals  are  quadrivalent  in  these  acids,  their  con- 
stitution may  be  expressed  by  the  general  formula — 

/° 
M<    | 
I  \Q    .          If  they  are  biyalent,  their  constitution 

OH   OH 

would  be  similar  to  that  of  sulphuric  acid.  As  far  as  the 
evidence  thus  far  in  our  possession  is  concerned,  we  are 
as  much  justified  in  accepting  one  of  these  formulas  as 
the  other. 

A  very  important  salt  of  chromium  is  that  known  as 
potassium  bichromate.  The  formula  of  this  salt  is 
Cr.2O.K2.  It  may  be  regarded  as  the  salt  of  an  acid 
which  is  analogous  to  pyrosulphuric  acid  and  derived 
from  chromic  acid  by  the  abstraction  of  water,  thus : — 


CrO 


OH 


CrOa< 


,OH 


.W       H20         = 

CrO 


OH 
OHj 

2  mol.  Chromic  acid.  Pyrochromic  acid. 

Neither  chromic  acid  itself  nor  pyrochromic  acid  can 
he  prepared  in  the  free  condition.  The  group  CrO,2  does 
not  appear  to  be  capable  of  holding  hydroxyl  in  combi- 


nation.     So  that  salts  of  the  formula  Cr02 — OM      are 
not  known. 

An  acid  of  manganese  furnishes  salts  of  the  general 
formula  Mn04M.  No  positive  assertion  can  be  made  in 
regard  to  the  constitution  of  this  acid,  except  that  it  is 
monobasic,  and  hence  it  probably  contains  one  hydroxyl 
group.  This  gives  the  formula  Mn03 — OH,  but  the 
group  Mn03  remains  unexplained. 

Compounds  of  Uranium. — In  connection  with  the  sub- 
ject of  bases  it  was  mentioned  that  uranium  forms  a 
peculiar  set  of  salts  in  which  the  monovalent  group  UO 
takes  the  place  of  the  hydrogen  of  the  acids.  This 
group  UO  is,  like  U  itself,  a  base  residue,  and  hence, 
16 


182  CHEMICAL    COMPOUNDS. 

according  to  the  general  definition  of  salts,  it  may  take 
the  place  of  hydrogen  in  the  acids,  and  the  resulting 
compounds  will  be  just  as  strictly  salts  as  those  com- 
pounds in  which  the  base  residue  consists  of  a  metal 
alone. 

Uranium,  further,  forms  salts  of  the  general  formula 
U  07M2,  which  may  be  supposed  to  be  derived  from  a 
complex  hydroxide,  U4O.H2  =  OU.O.U(OH).O.UO. 
This  formula  is,  however,  only  hypothetical. 


CONSTITUTION  OF  CARBON  COMPOUNDS. 

As  has  already  been  intimated,  a  great  deal  more  is 
known  concerning  the  constitution  of  carbon  compounds 
than  is  known  concerning  the  constitution  of  those  com- 
pounds which  do  not  contain  carbon.  Having  considered 
the  general  constitution  of  the  classes  of  compounds 
with  which  we  meet,  it  only  remains  to  study  those 
changes  which  the  members  of  the  different  classes  can 
undergo  without  losing  their  main  characteristics.  We 
shall  find  that  the  compounds  of  carbon  may  be  divided 
into  a  few  distinct  groups  ;  that  each  of  these  groups 
possesses  a  mother-substance  from  which  all  the  other 
members  of  the  group  may  be  derived.  The  principal 
groups  are :  the  Marsh-gas,  or  Methane  compounds,  also 
called  Fatty  Bodies ;  the  Benzene  compounds,  also  called 
Aromatic  Bodies  ;  the  Naphthalene  compounds  ;  and  the 
Anthracene  compounds.  The  first  two  groups  comprise 
by  far  the  largest  number  of  carbon  compounds. 


,  METHANE  DERIVATIVES.    (FATTY  BODIES.) 

First  Group. 
Bodies  derived  from  the  Hydrocarbons  CnH2n-\-2. 

The  constitution  of  methane  has  been  considered  above 
(see  ante,  p.  125).  It  was  also  shown  that  by  the  linking 
of  carbon  atoms  to  each  other  the  possibility  is  given  for 


CONSTITUTION    OP    CHEMICAL    COMPOUNDS.        183 

the  formation  of  an  homologous  series,  the  members  of 
which  differ  from  each  other  by  CH>2,  or  a  multiple  of 
this.  The  following  members  of  this  series  have  been 
particularly  well  studied. 

Methane,  CH,.  Pentane,  C5H12. 

Ethane,  C2H6.  Hexane,  C6H14. 

Propane,  CsHb.  Heptane,  C7H16. 

Butane,  C4H10. 


In  speaking  of  substitution  products  it  was  stated  that 
only  one  mono-substitution  product  of  methane  could 
exist,  according  to  the  views  now  held  concerning  consti- 
tution. The  same  thing  is  true  of  other  substitution 
products  in  which  more  than  one  substituting  group  is 
present.  Further,  we  can  only  conceive  of  one  variety 
of  methane  itself,  and  only  one  variety  has  ever  been 
observed. 

Derivatives  of  Ethane,  C>2H6. — Only  one  variety  of  this 
hydrocarbon  can  exist,  and  only  one  variety  has  been 
observed.  Of  its  mono-substitution  products  also,  only 
one  variety  can  exist,  and  only  one  variety  has  been 
observed. 

Of  the  bi-substitution  products,  however,  two  varieties 
are  possible,  as  may  be  seen  by  comparing  the  following 
formulas : — 

H    H  H   X 

I      I  II 

X— C— C— X  and  H— C— C— X   . 

H    H  H    H 

In  the  first,  the  substituting  groups  are  in  combination 
with  different  carbon  atoms ;  in  the  second,  both  substi- 
tuting groups  are  in  combination  with  the  same  carbon 
atom. 

A  number  of  compounds  are  known  belonging  to  the 
classes  of  which  these  are  the  general  formulas.  X  may 
represent  any  of  the  substituting  groups  with  which  we 
have  had  to  deal;  or  the  class  groups  CH2OH,  COH, 
COOH,  etc. 


184  CHEMICAL    COMPOUNDS. 

The  simplest  of  these  are  the  chlorides,  one  of  which 
is  CHC12.CHH,  and  the  other  CHseCl.CH2Cl.  The  first 
is  called  ethylidene  chloride,  the  second,  ethylene  chlo- 
ride. The  constitution  of  these  compounds  follows  from 
the  following  facts : — 

Ethylidene  chloride  is  produced  by  the  action  of  phos- 
phorus pentachloride  on  aldehyde.  We  have  seen  that 

/H 

an  aldehyde  contains  the  group      — C=0    '      Ordinary 

/H 

aldehyde  is       CH, — C=0    '      As,  in  the  reaction  with 
phosphorus  chloride,  the  oxygen   is  simply  replaced  by 
chlorine,  the  constitution  CH3 — CHC12  follows  for  ethyl- 
idene chloride.     As  a  consequence,  the  formula — 
CHaCl.CH2Cl  must  be  that  of  ethylene  chloride. 

Other  compounds  closely  related  to  these  two  chlorides 
will  be  considered  under  the  heads  of  ethylene,  bibasic 
acids,  etc. 

Derivatives  of  Propane,  C.AHk. — Propane  may  be  con- 
sidered as  a  mono-substitution  product  of  ethane,  derived 
from  the  latter  by  replacing  an  atom  of  hydrogen  with 
CH3.  From  what  was  said  above,  it  will,  hence,  be  seen 
that  only  one  variety  of  propane  can  exist;  only  one 
variety  has  been  observed. 

Under  the  head  of  substitution  products,  it  has  been 
.shown  that  there  are  two  kinds  of  carbon  atoms  and, 
consequently,  two  kinds  of  hydrogen  atoms  in  propane 
(which  see) ;  and  hence,  further,  that  two  different  mono- 
substitution  products  may  be  obtained  from  this  hydro- 
carbon. These  have  the  general  formulas : — 

H   H    H  H    X    H 

X— C— C— C— H  and  H— C— C— C— H    . 

H    H    H  H    H    H 

The  compounds  represented  by  the  first  formula  are 
known  as  propyl  compounds;  those  represented  by  the 
second  formula  as  isopropyl  or  pseudopropyl  compounds. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        185 

The  two  alcohols,  normal  propyl  alcohol — 
CH3.CH.,.CH2OH,  and  pseudopropyl  alcohol— 
CPTj.CHOH.CHg,  are  the  starting-points  for  the  prepara- 
tion of  the  two  series  of  isomeric  propyl  compounds. 
As  the  former  is  a  primary  alcohol,  it  follows,  from  what 
has  been  said  concerning  these  alcohols,  that  it  must  con- 
tain the  group  CH2OH.  This  it  can  only  do  if  the 
hydroxyl  group  is  in  combination  with  one  of  the  ter- 
minal carbon  atoms.  Consequently,  the  above  constitu- 
tion is  assigned  to  it.  By  replacing  the  hydroxyl  by 
chlorine,  bromine,  iodine,  C3ranogen,  etc.,  corresponding 
derivatives  are  obtained. 

Pseudopropyl  alcohol  is  obtained  from  acetone  and, 
being  a  secondary  alcohol,  contains  the  group  CH.OH. 
Consequently,  its  hydroxyl  is  in  combination  with  the 
central  carbon  atom  of  propane.  By  replacing  the  hy- 
droxyl with  chlorine,  bromine,  iodine,  cyanogen,  etc.,  cor- 
responding pseudopropyl  derivatives  are  obtained. 

Derivatives  of  Butane,  G4H10. — Butane  may  be  con- 
sidered as  a  mono-substitution  product  of  propane,  con- 
sequently, two  varieties  must  be  possible,  one  of  which 
would  have  the  formula — 

H    H    H    H 

I.  H— C— C— C— C— H    ; 

I       I       I       I 
H    H    H     H 

while  the  other  would  have  the  formula — 

PI    H    H 
I       I       I 

II.  H— C— C— C— H 


H  HH 

As  a  matter  of  fact,  two  varieties  of  butane  are  known 
to  us,  viz.,  normal  butane  and  trimethylmethane.  The 
former  has  the  constitution  represented  by  formula  I. 
above ;  the  latter  that  represented  bv  formula  II. 

10* 


186  CHEMICAL    COMPOUNDS. 

Proofs. — The  proof  of  the  formula  of  normal  butane 
is  the  same  in  nature  as  that  given  for  ethane  (see  p.  127). 
It  is  formed  by  the  action  of  zinc  or  sodium  on  eth}-l 
iodide,  according  to  the  equation : — 

HH  HH  HHHH 

H-C-C— I    +     1— C-C-  H    +     Zn      =    H— C-C-C— C-H    +     ZnI2  . 

HH  HH  HHHH 

lodethane.  Normal  butane. 

Of  course,  we  here  assume  that  we  know  the  formula 
of  iodethane,  but  we  have  already  presented  good 
grounds  for  this  assumption.  Starting  with  this  formula, 
we  are  led  very  easily  to  the  above  formula  of  normal 
butane. 

Trimethylmethane  is  obtained  from  psendobutyl  iodide, 
the  constitution  of  which  is  known  to  be — 

CH3\ 

J>CI — CH3  .      When  the  iodine  is  replaced  by  hy- 


drogen,  the  hydrocarbon  is  the  product.     (See  Pseudo- 
butyl  Alcohol.) 

Of  normal  butane,  two  kinds  of  simple  substitution 
products  are  possible,  of  the  general  formulas: — 

H    H    H  H    H     X    H 

III  I      I       I       I 

I.  H— C— C— C— C— X    and  II.  H— C— C— C— C— H 

I       I       I       I  I       I       I       I 

HHHH  HHHH 

Of  trimethylformene,  there  are  also  two  kinds  possible, 
of  the  formulas — 

H    H    H  H    X    H 

III  III 

III.     X— C— C— C— H   and     IT.     H— C— C— C— H  . 

Ill  II 

H    C     H  H     C 


/1\  / 

HH  H  HI 


l\ 
HH 

Representatives  of  all  four  kinds  of  substitution  pro- 
ducts are  known.    The  principal  of  these  are  the  alcohols. 

1.  Normal  butyl  alcohol,  CH,CH9.CH2.CH2  OH. 

2.  Secondary  butyl  alcohol,  CH3  CH.OH.CH^CH,. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        187 

GEL 
3.  Isobutyl  alcohol,  >CH.CH2.OH. 


CH 
4.  Tertiary  butyl  alcohol,  CI^.C.OH/       * 


From  each  of  these  alcohols  the  corresponding  chlo- 
rides, bromides,  etc.,  can  easily  be  obtained. 

Proofs. — Normal  butyl  alcohol  is  obtained  indirectly 
from  normal  butyric  acid,  the  constitution  of  which  is 
known. 

Secondary  butyl  alcohol  is  converted  by  oxidation  into 
ethyl-methyl-acetone.  C2H5 — CO — CH.V  It  is,  hence,  a 
secondary  alcohol,  and  its  constitution  is  that  expressed 
above. 

Isolwtyl  alcohol  is  converted  into  isobutyric  acid  by 
oxidation.  The  constitution  of  the  acid  is  known,  and 
hence  also  that  of  the  alcohol. 

Tertiary  butyl  alcohol  is  a  tertiary  alcohol,  and  hence 
contains  the  group  COH.  It  is  prepared  by  treating 
acetyl  chloride,  CH3.COC1,  with  zinc  methyl,  Zn(CH3)a, 
and  hence  contains  three  groups  CH..  The  only  for- 
mula which  is  in  accordance  with  these  facts  is  that  above 
assigned  to  the  alcohol. 

Derivatives  of  Pentane,  C^H^. — Three  varieties  of 
pentane  may  exist.  These  have  the  formulas : — 

1.  H.C.CH^.CH^.CH^CH,. 

CH3 

2.  H3C.CH2.CH<; 


3.  0- 


All  three  of  these  compounds  are  known.  The  first  is 
normal  pentane  ;  the  second  is  ethyldimethyl  methane; 
and  the  third  tetramethyl  methane. 

Proofs.  —  Normal  pentane  is  obtained  by  replacing  the 
CN  group  of  the  cyanide  of  normal  butane  by  hydrogen. 


188  CHEMICAL    COMPOUNDS. 

We  have  seen  above  how  the  formula  of  the  cyanide  itself 
is  determined. 

Ethyldi  methyl  methane  is  derived  from  ordinary  amyl 
alcohol,  and  hence  has  the  same  general  constitution. 
The  proofs  for  the  constitution  of  this  alcohol  will  be 
given  below. 

Tetramethyl  methane  is  derived  from  the  iodide  of 
tertiary  butyl  alcohol  by  the  action  of  zinc  methyl.  The 
reaction  takes  place  as  follows  :  — 


CH3.CI< 

\CH3 

/CH3 

rn/ 

CH3 

jll\ 

XCH3 

CH..CI/ 

Iodide  of  tertiary 
butyl  alcohol. 

C 


Tetramethyl  methane. 


A  great  variety  of  substitution  products  can  he  ob- 
tained from  the  isomeric  butanes.  Of  the  alcohols  five 
are  known,  as  follows:  — 

1.  Normal  amyl  alcohol,  CH3.CH  .CHa.CH2.CHaOH. 


2.  Amyl  alcohol  of  fermentation,  >CH.CELCH2OH. 

CH/ 

3.  Isoamyl  alcohol,  CH  .CH2.CH2.CHOH.CH3. 


4.  Amylene  hydrate,  >CH.CH.OH.CH3. 

CH 


. 

5.  Tertiary  amyl  alcohol,  >C.OH.CH2.CH3. 

CH/ 

These  alcohols,  like  the  others  which  have  been  con- 
sidered, form  the  starting-points  for  the  preparation  of 
corresponding  substitution  products. 

Proofs.  —  Normal  anv^l  alcohol  is  obtained  from  normal 
valeric  acid,  and  yields  this  acid  by  oxidation.  The  con- 
stitution of  the  acid  follows  from  its  method  of  prepara- 
tion. (See  Normal  Valeric  Acid.) 

Amyl  alcohol  of  fermentation  is  obtained  from  ordinary 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        189 

valeric  acid  by  reduction,  and  is  converted  into  this  acid 
by  oxidation.  Hence  its  constitution  is  similar  to  that 
of  the  acid  (which  see). 

Isoamyl  alcohol  is  a  secondary  alcohol  obtained  from 
methyl  propylketone.  The  constitution  of  the  latter 
being  given,  that  of  the  alcohol  follows. 

Amylene  hydrate  is  a  secondary  alcohol,  the  con- 
stitution of  which  is  believed  to  be  represented  by  the 
above  formula.  Good  proofs  are  lacking. 

Tertiary  amyl  alcohol  is  formed  from  propionyl  chlo- 
ride and  zinc  methyl,  and,  being  tertiary,  must  contain 
the  group  COH.  The  only  formula  which  is  in  harmony 
with  these  facts  is  that  given  above. 

Derivatives  of  Hexane,  C6HU.  —  Three  varieties  of 
hexane  are  known.  These  are 

1.  Normal  hexane,     H3C.CH2.CH2.CH2.CH2.CH3. 

'      CH. 

2.  Ethvl  isobutane,     H3C.CH2.CH2.CH 

H3CX  CH3 

3.  Diisopropane,  >CH.CH<; 

H3CK  \CH3 

Proofs.  —  Proofs  for  the  first  formula  are  wanting. 
The  constitution  of  ethyl  isobutane  follows  from  its 
method    of  preparation,   which    consists   in    treating    a 
mixture  of  iodethane  and  iodisobutane  with  sodium. 
lodisobutane,  being  obtained   from  isobutyl   alcohol, 

CH 
has  the  constitution  ;>CH.CH  I   .     Hence  the   rc- 

CH/ 
action  may  be  represented  thus:  — 


)CH.CHaI     +     ICH,.CH3     -f     2Na     = 
CH 


lodidobutane.  Iodethane. 


CH3X 

CH.CH2.CK,CH3     -f     2N 


CH 


Etliyl  isobutane. 


190  CHEMICAL    COMPOUNDS. 

Diisopropane   is   obtained   by  treating   iodisopropane 
with  sodium.     This  iodide,  being  derived  from  isopropyl 

H3CX 
alcohol,  has  the  constitution  /CHI  .      Hence  the 

HiG/ 
reaction  is 

H3<\  /CH3 

>CHI     -f     ICH<  +     2Na    = 

H.CK  XCH3 

H3CX  /CH3 

CH— HC 


Five  alcohols  are  known  which  are  derived  from  these 
varieties  of  hexane.     They  are : — 

1.  Primary  hexyl  alcohol,  H3C.CH2.CH2.CH2.CH2.CH2OH. 

2.  Secondary  hexyl  alcohol,  H3C.CH2.CH2.CH2.CH.OH.CH3. 

H3(\ 

3.  Dimethyl  propyl  carbinol,  )C.OH.CH2.CH2.CH3. 

TT    f\^ 


H3C.CR, 


\, 


4.  Diethyl  methyl  carbinol,  >C.OH.CH3. 

H3C.CH./ 

CH3  CH3 

5.  Dimethyl  pseudopropyl  carbinol,  ;C.OH.CH; 


Proofs.  —  Good  proofs  for  the  first  formula  are  wanting. 

Secondary  hexyl  alcohol  yields  by  partial  oxidation 
methyl  butyl   ketone.     It  is   consequently  a  secondary 
alcohol,  and  contains  the  groups  CH3  and 
CH3.CH2.CH2.CH2.     This  gives  the  above  formula. 

Dimethyl  propyl  carbinol  is  obtained  from  butyryl 
chloride  and  zinc  methyl. 

Diethyl  methyl  carbinol  is  obtained  from  acetyl  chlo- 
ride and  zinc  ethyl. 

Dimethyl  pseudopropyl  carbinol  is  obtained  from 
isobutyryl  chloride  and  zinc  metlryl. 

Derivatives  of  Heptane,  C.H^.  —  Three  varieties  of 
heptane  are  known,  one  of  which  is  probably  normal 
heptane. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        191 

A  second  varietj',  ethyl-a*rnyl, 

/CH, 
H3C.CH2.CH2.CH2.CH<  , 

XCH3 

is  obtained  by  the  action  of  sodium  on  a  mixture  of 
ethyl  and  amyl  iodides,  the  latter  from  ordinary  amyl 
alcohol.  Ordinary  amyl  alcohol  is 


>CH.CEL.CEL.OH  .     Consequently  the  iodide  is 
CH/ 
. 

^>CH.CH2.CH2I  .      The  reaction  is  represented  by 
/ 
the  following  equation  :  — 

CEU 

>CH.CH2.CH2I  +  ICHa.CH8  +  Na2  = 
CH/ 


)CH.CH2.CH2.CH..CH3  +  2NaI 
CH 


A  third  variety  of  hexane  is  dimetliyldietliylmethane, 
H3CX     XCH2.CH3 

^>C^  .       This    is    obtained    from    zinc- 

H3CK      \CH2.CH3 
ethyl  and  acetone  chloride,  thus  :  — 

CH3 
C,H5X  _        I  C2H5X       /CH, 

>|Zn     +     ClJC      =  >C<  +ZnCL  . 

C2H/  ^  |  C2H/     \CH3 

CH3 

Zinc-ethyl.  Acetone  Dimetbyldiethyl- 

chloride.  methane. 

Acetone  chloride,  being  produced  by  the  replacement  of 
the  oxygen  of  acetone  with  chlorine,  must  have  the  above 
constitution. 

Some  of  the  alcohols  corresponding  to  these  hydro- 
carbons are  known.  The  constitution  of  these  is  readily 
understood  so  soon  as  we  know  the  methods  of  their 
formation. 


192  CHEMICAL    COMPOUNDS. 

Not  much  is  known  concerning  the  constitution  of  the 
remaining  hydrocarbons  of  the  methane  series  or  their 
derivatives. 


Monobasic,  Monatomic  Acids,  C^H^nOv  —  The  acids  of 
this  series  may  be  considered  as  substitution  products  of 
the  hydrocarbons  formed  by  replacing  a  hydrogen  atom 
of  the  latter  with  carboxyl  (COOH).  In  most  cases, 
these  acids  have  been  prepared  by  converting  the  group 
CN  of  the  cyanides  of  hydrocarbon  residues  into  COOH. 
If  we  then  know  the  constitution  of  the  cyanide,  the  con- 
stitution of  the  acid  is  readily  deduced. 

The  principal  members  of  the  series  are 

Formic  acid,  H.COOH. 

Acetic  acid,  CH3.COOH. 

Propionic  acid,  C2H5.COOH. 

Butyric  acid,  C3HrCOOH. 

Valeric  acid,  C4H9.COOH. 

Caproic  acid,  C5HU.COOH. 

Of  formic  acid  and  its  substitution  products  only  one 
variety  is  known. 

Of  acetic  acid  and  its  substitution  products,  also,  only 
one  variety  is  known. 

Propionic  Acid  —  With  propionic  acid  the  case  is 
different.  Of  the  acid  itself  only  one  variety  is  known, 
but  of  the  mono-substitution  products  two  varieties  are 
known.  The  constitution  of  the  acid  is 

H    H 

H—  C—  C—  COOH  .      Now   it   is    plain   that,   in   this 

I       I 
H    H 

compound,  aside  from  the  hydrogen  of  the  carboxyl 
group,  there  are  two  kinds  of  hydrogen  atoms  —  those 
combined  with  a  carbon  atom  which  in  its  turn  is  in 
combination  with  the  group  CH2;  and  those  in  combina- 
tion with  a  carbon  a.tom  which  in  its  turn  is  in  combina- 
tion with  two  carbon  atoms.  The  case  is  similar  to  that 
of  propane,  of  which  we  saw  that  two  varieties  of  sub- 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS         193 

stitution  products  are  possible.     Here  we  have  the  two 
possibilities  expressed  by  the  formulas 

H     X  H     H 


II—  C—  C—  CO.OH  and         X—  C—  C—  COOH  . 

ii  L  A 

Products  of  the  first  kind  are  designated  as  a-substitution 
products  ;  those  of  the  second  kind  as  3-substitution 
products.  The  best  representatives  of  these  two  classes 
of  compounds  are  the  two  lactic  acids  (which  see). 
Lactic  acids  are  derived  from  propionic  acid  by  replacing 
a  hydrogen  atom  of  the  latter  with  hydroxyl.  One  of 
the  two  lactic  acids  is  obtained  indirectly  from  ethylene, 
which  will  be  shown  to  have  the  formula 
CH2—  CH2 

|  or    ||        .To  this  body  hypochlorous  acid  may 

CH2—         CH2 
be  added,  and  the  constitution  of  the  resulting  compound 

CH2.OH 
is      |  .      From  this,  by  treatment  with  potassium 

CHrCl 

cyanide,  KCN,  is  obtained  the  corresponding  cyanide, 
which,  when  boiled  with  alkalies,  yields  lactic  acid  of  the 

H     II 

I       I 
constitution      HO  —  C  —  C  —  CO.OH.     Now,  by  replacing 


i 


the  hydroxyl  group  of  this  acid  with  chlorine,  bromine, 
etc.,  |3-mono-substitution  products  are  formed.  The 
isomeric  compounds  of  the  a-series  are  those  which  can- 
not be  prepared  in  the  manner  described,  and  are  of  the 
same  composition  as  those  obtained  from  3-lactic  acid. 

Butyric  Acids.  —  Two  acids  of  the  formula  C3H7.COOH 
are  theoretically  possible,  and  two  are  known.  These 
are  normal  butyric  acid,  CH3.CH2.CH2.COOH,  and 

CH3X 
isobutyric  acid,  >CH.COOH  . 

OH/ 
It 


194  CHEMICAL    COMPOUNDS. 

Proof*.  —  Normal  butyric  acid  is  prepared  by  intro- 
ducing the  group  C2H5  into  acetic  acid,  thus  :  — 

CH2NaCOOH   -f   C2H5I   =  C2H5.CH2.COOH   +  Nal. 

Further,  by  reduction,  normal  butyric  acid  yields  one 
of  the  two  possible  primary  butyl  alcohols.  It  was 
shown  (p.  18t)  that  the  other  possible  primary  butyl 
alcohol  is  not  a  derivative  of  normal  butane;  conse- 
quently normal  butyric  acid  must  be  derived  from 
normal  butane,  and  it  has  the  formula  above  assigned 
to  it. 

Isobutyric  acid  is  obtained  from  pseudopropyl  c}ranide 
(p.  185),  and  this  has  been  shown  to  have  the  constitution 
H  CN  H 

H—  C  —  C  --  C—  H  . 

I        I         I 
H      H      H 

From  this  the  above  constitution  follows  for  isobutyric 
acid. 

Valeric  Acids.  —  Two  are  well  known,  viz.,  normal 
valeric  acid,  CHH.CH2.CH2.CH2.COOH,  and  ordinary 


valeric  acid,  >CH.CH2.COOH  . 

CH/ 

Proofs.  —  The  cyanide  from  normal  butyl  alcohol  must 
have  the  formula  CH3.CH2.CHS9.CH2.CN.  *  By  conversion 
of  the  CN  group  into  COO  H,,  this  cyanide  yields  normal 
valeric  acid. 

The   cyanide   from   isobutyl   alcohol   must    have   the 

CH3X 
formula  >CH.CH2.CN  .  This  yields  ordinary 

CH/ 
valeric  acid. 

Caproic  Acids  __  Four  of  these  acids  are  known.  They 
are  normal  caproic  acid,  CH3.CH2.CH2.CH2.CH2.COOH; 


ordinary    caproic    acid.  >CH.CH2.CH2.COOH 

CH/ 


CONSTITUTION    OP    CHEMICAL    COMPOUNDS.        195 

CH,  CH, 

isocaproic    acid*  )>CH.CH<^  ;  and 

CH/  \CO.OH 

C,H6\ 

pseudocaproic   acid,  /CH.COOH  . 


Proofs.  —  The  cyanide  from  normal  amyl  alcohol  yields 
normal  caproic  acid. 

Ordinary  caproic  acid  is  obtained  from  the  cyanide  of 
ordinary  amyl  alcohol. 

Isocaproic  acid  is  prepared  from  the  cyanide  corre- 
sponding to  amylene  hydrate. 

Pseudocaproic  acid  is  obtained  by  introducing  two 
groups,  C2H.,  into  acetic  acid,  thus:— 

CHNa..COOH     +     2(OaHJ)     = 
P  TT 

5\CH.COOH     +     2NaI  . 
0,H/ 


The  other  acids  of  this  series  are  not  very  well  known. 
By  the  aid  of  the  foregoing  examples,  the  method  of 
determining  the  constitution  of  the  known  acids  will  be 
readily  understood. 

Aldehydes. — Corresponding  to  every  primary  alcohol 
and  to  every  acid  there  is  an  aldehyde.  The  constitution 
of  each  of  these  aldehydes  is  given  if  we  know  from  which 
acid  or  from  which  alcohol  it  is  obtained. 

The  aldehydes  are  produced  from  the  primary  alcohols 
by  partial  oxidation ;  and  from  the  acids  by  subjecting  a 
mixture  of  a  salt  of  the  acid  and  a  salt  of  formic  acid  to 
dry  distillation. 

Acetones  or  Ketones. — The  ketones  are  obtained  by 
distilling  mixtures  of  two  acids.  If  the  constitution  of 
the  acids  is  known,  that  of  the  ketone  obtained  in  each 
case  is  also  known. 


196  CHEMICAL    COMPOUNDS. 

Second  Group. 
Bodies  obtained  from  the  Hydrocarbons  CnH2n. 

If  carbon  is  always  quadrivalent,  then  the  members  of 
this  series  of  hydrocarbons  are  either  nnsaturated,  or  in 
them  the  carbon  atoms  are  united  b}7"  more  than  one 

CH,- 

afflnity  each.     Thus  ethylene,  C2H4,  is  either  , 

CH- 

CH2 
an  unsaturated  compound,  or  it  is  ,    in  which  the 

CH2 

two  carbon  atoms  are  united  by  means  of  two  affinities 
each.  Up  to  the  present  no  proofs  have  been  given  for 
either  of  these  formulas.  In  regard  to  these  hydrocar- 
bons, we  only  know  that  they  easily  take  up  two  atoms 
of  monovalent  elements. 

Ethylene  and  Derivatives. — In  connection  with  ethane 
derivatives  it  was  stated  that  two  chlorides  are  known, 
both  of  which  have  the  formula  C2H4C12.  One  of  these 
is  obtained  from  aldehyde  by  replacing  the  oxygen  atom 
with  two  chlorine  atoms ;  hence  its  formula  was  assumed 
to  be  CHCla.CHs.  The  isomeric  compound  has  the 

CH.C1  ' 
formula       | 

CH2C1 

This  latter  compound  is  obtained  from  ethylene  by 
direct  addition  of  chlorine,  whence  it  is  concluded  that 
ethylene  itself  is  symmetrical,  i.  e.,  that  each  carbon 
atom  in  it  holds  in  combination  two  hydrogen  atoms, 

CH3_ 
giving  the  constitution  expressed  by  the  formula 

CH2- 
CH2 
or      || 

OH, 
A   number  of  products  are  known   corresponding  to 

CH2C1 

ethylene  chloride,    '  ,  among  which  may  be  men- 

CH2C1 

tioned  the  bromide,  iodide,  and  cyanide.  By  replacing 
the  chlorine  or  bromine  of  ethylene  chloride  or  bromide 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        197 

with  hydroxyl,  an  alcohol  is  obtained  of  the  constitution 

CH2OH 

|  ,  which  is  the  simplest  representative  of  the 

CH2OH 
diatomic  alcohols  or  glycols. 

Propylene,  etc. — The  remaining  hydrocarbons  of  this 
series  are  obtained  for  the  most  part  by  treating  the 
chlorides,  bromides,  or  iodides  of  the  hydrocarbons  of 
the  methane  series  with  alcoholic  potassa,  by  which 
means  C1H,  BrH  orlH  is  abstracted  from  the  compound. 
Thus,  from  C3H7I  we  obtain  C3H6:  from  C4H9l  we  obtain 
C4H8,  etc. 

Jn  many  cases  the  method  of  formation  of  the  hydro- 
carbon leads  us  directly  to  its  constitution.  In  some 
cases  a  doubt  exists  even  after  all  the  methods  of  forma- 
tion and  the  products  of  decomposition  are  taken  into 
consideration. 

Alcohols. — Theoretically,  a  series  of  alcohols  is  possible, 
derived  from  the  hydrocarbons  of  the  ethylene  series  by 
the  replacement  of  one  hydrogen  atom  with  one  hydroxyl 
group.  Only  one  such  alcohol  is  known.  This  is  allyl 
alcohol,  C3H5.OH,  or  CH2=CH.CH2.OH. 

Proofs. — Allyl  alcohol  differs  from  propyl  alcohol  in 
containing  two  hydrogen  atoms  less.  Now,  by  treating 
allyl  alcohol  with  nascent  hydrogen,  it  is  converted  into 
normal  propyl  alcohol,  which,  as  we  have  seen,  has  the 

H    H    H 

I       I       I 
constitution      H — 0 — C — C — OH  .        Hence   it   is   as- 

I       I       I 
H    H    H 

sumed  that  in  allyl  alcohol,  as  well  as  in  propyl  alcohol, 
the  hydroxyl  is  in  combination  with  one  of  the  terminal 
carbon  atoms,  and,  accordingly,  it  must  be  either — 
CH2  CH3 

II  1 

CH  or     CH         .If  the   second   formula   were 


CH,OH  CHOH 


17* 


198  CHEMICAL    COMPOUNDS. 

correct  we  should  expect  allyl  alcohol  to  yield  acetic 
acid  by  oxidation,  inasmuch  as  it  contains  the  group 
CH3  in  combination  with  another  carbon  atom.  Not  a 
trace  of  acetic  acid  is  formed,  however,  and  hence  the 
first  of  the  two  formulas  above  given  is  usually  accepted. 
The  proofs  for  this  formula  are  not  positive. 

By  replacing  the  hydroxyl  of  altyl  alcohol  with  chlo- 
rine, bromine,  iodine,  cyanogen,  etc.,  the  corresponding 
chloride,  bromide,  iodide,  and  cyanide  are  obtained. 

Acids. — Though  allyl  alcohol  is  primary,  it  cannot  be 
directly  oxidized  to  form  a  corresponding  acid.  But  if 
the  alcohol  is  first  combined  with  bromine  and  then  oxi- 
dized,  a  bibrompropionic  acid  is  obtained  which,  when  again 
freed  of  bromine,  yields  acrylic  acid.  These  reactions 
strengthen  the  conclusion  above  drawn,  viz.,  that  allyl 
alcohol  contains  the  group  CH^OH.  The  reactions  are— 

CH2  CH2Br 

II  I 

1.  CH  +       Br2  CHBr    . 

CH2OH  CH2OH 

CH2Br  CH2Br 

2.  CHBr       by  oxidation   yields     CHBr      . 

CH2OH  COOH 

CH2Br  CH2 


3.      CHBr      -f     Zn      =      CH  -f     ZnBr,  . 

COOH  COOH 

From  these  changes  we  are  led  to  the  formula — 
CH3 

CH         for  acrylic  acid.     This  acid  is  the  first  of  a  series 

COOH 

each  of  which  differs  from  the  corresponding  member  of 
the  series  Cn  1X2^02  by  containing  two  hydrogen  atoms 
less. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        199 

The  member  succeeding  acrylic  acid  in  this  series  is 
crotonic  acid,  to  which  the  constitutional  formula — 
CH2=CH— CH2—  CO.OH    is   usually   assigned.       The 
grounds  for  this  are  as  follows : — 

Allvl  cyanide,  as  has  been  shown,  has  the  formula 
CH2=CH — CH2.CN.  This  cyanide,  when  properly 
treated,  is  converted  into  crotonic  acid. 

When  treated  with  nascent  hydrogen,  it  yields  normal 
butyric  acid,  which  shows  that  the  carboxyl  group  of 
crotonic  acid  is  in  combination  with  one  of  the  terminal 
carbon  atoms. 

Similar  considerations  lead  to  a  knowledge  of  the  con- 
stitution of  the  remaining  members  of  this  series.  None 
of  these,  however,  have  been  investigated  as  fully  as 
acr3'lic  and  crotonic  acids. 

Third  Group. 
Bodies  derived  from  the  Hydrocarbons  Cn  Hin—z. 

CH2Br 

When  ethylene  bromide,      |  ,  is  heated  with  al- 

CH2Br 

coholic  potassa,  two  molecules  of  bydrobromic  acid  are 
given  off  and  a  compound  of  the  formula  C.,H2  is  pro- 
duced, which  is  the  first  of  a  series  of  similar  hydro- 
carbons. Just  as  ethylene  must  be  considered  either  as 
unsaturated  or  as  having  its  carbon  atoms  combined  by 
the  action  of  more  than  one  affinity  of  each  atom,  so 
also  with  the  hydrocarbon  C.2H2,  or  acetylene.  In  the 
latter  case,  however,  if  the  compound  is  unsaturated, 
each  carbon  atom  must  be  united  by  means  of  three 
affinities  each.  The  formula  of  acetylene  is,  accordingly, 

=rCH  CH 

|      ,     or     HI       .      The  latter  formula  is  usually  ac- 
=CH  CH 

cepted,  though  the  grounds  for  it  are  weak.  However, 
whether  this  treble  union  exists  between  the  carbon 
atoms  or  not,  we  can  be  moderately  certain  .that  acety- 
lene, like  eth%ylene,  is  symmetrically  constructed,  i.  e., 
that  each  carbon  atom  is  in  combination  with  one  hydro- 
gen atom.  This  follows  from  the  fact  that  acetylene  is 


200  CHEMICAL    COMPOUNDS. 

formed  by  abstracting  hydrobromic  acid  from  ethylene 
bromide.      For  the   latter   compound    has    the   formula 

GH,Br 

,    and  it  appears  most  probable  that  the  splitting 
CH.2Br 
off  of  BrH  would  take  place  as  follows : — 


CH 
CH 


HBr  CH  CH 

—     2HBr       =       I         or     ||| 
HBr  CH  CH 


Acetylene  and  its  homologues  have  this  common  pro- 
perty, they  each  combine  with  four  atoms  of  chlorine  or 
bromine,  thus  forming  saturated  compounds,  which  may 
be  regarded  as  substitution  products  of  the  hydrocarbons 
of  the  marsh-gas  series.  Very  little  is  known  regarding 
the  constitution  of  the  higher  members  of  the  series. 
No  alcohols  are  known  corresponding  to  the  above 
hydrocarbons. 

A  few  acids  have  been  studied,  the  general  formula  of 
which  is  CwH2n_402,  which  may  be  considered  as  derived 
from  the  hydrocarbons  of  the  acetylene  series  by  replacing 
a  hydrogen  atom  with  carboxyl.  Their  constitution  is 
in  no  case  well  known. 

Fourth  Group. 
Diatomic  Alcohols  and  Acids. 

It  has  been  stated  that  when  we  replace  the  bromine  of 
ethylene  bromide  with  hydroxyl,  an  alcohol  of  the  con- 

CH2.OH 
stitution      |  is  obtained.     Alcohols  of  this  kind 

CH2.OH 

which  contain  two  hydrogen  groups  are  called  diatomic 
alcohols  or  glycols.  The  best  studied  diatomic  alcohol  is 
the  one  the  constitutional  formula  of  which  is  given 
above.  This  is  ethylene  alcohol  or  ethylglycol.  If  the 
above  formula  is  correct,  we  are  justified  in  expecting 
that  ethylene  alcohol  will  yield  two  products  by  oxida- 
tion. The  first  would  be  formed  if  only  one  of  the 
groups  CH^OH  were  converted  into  CO  OH.  It  would 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        201 

CH.2OH 

have  the  constitution      |  .    The  second  would  be 

COOH 

formed  if  both  the  groups  CH2OH  were  converted  into 

COOH 

COOH.     It  would  be      |  .      Both  these  products 

COOH 

are  actually  known,  and  may  be  obtained  by  the  oxida- 
tion of  ethylene  alcohol.  They  are  both  representatives 
of  new  classes  of  compounds,  the  nature  of  which  may 
be  easily  understood. 

CH^OH 

The  compound      |  ,      or   glycolic   acid,  is   half 

COOH 

alcohol  and  half  acid,  and  it  can  be  shown  to  possess 
the  properties  of  both.  It  is  acetic  acid  in  which  one 
hydrogen  atom  has  been  replaced  by  hjTiroxyl,  or  oxy- 
acetic  acid.  The  acids  of  which  it  is  the  representative 
are  known  as  oxyacids. 

COOH 

The  compound       |  is  an  acid,  and,  from  what 

COOH 

was  said  concerning  acids  in  general,  it  will  be  recognized 
as  a  bibasic  acid.  It  is  known  as  oxalic  acid.  It  is  the 
first  of  a  series  of  bibasic  acids. 

Diatomic  Alcohols,  GnHZnJrZOv — Ethylene  alcohol  is 
the  only  member  of  this  series  that  is  well  known.  In 
regard  to  its  constitution  enough  has  been  said  above  to 

CH3OH 

show  upon  what  grounds  the  formula  rests. 

CH..OH 


We  may  obtain  a  great  variety  of  derivatives  from  this 
alcohol  by  replacing  one  or  both  of  its  hydroxyl  groups- 
with  monovalent  elements  or  groups.  The  products 
obtained  by  the  addition  of  various  elements  or  groups 
to  ethylene  may  also  be  considered  as  derivatives  of  ethy- 
lene alcohol. 


202  CHEMICAL    COMPOUNDS. 

Diatomic  Acids,    CnH2nOt. — The  acids  of  which  gly- 

CH,OH 
colic  acid,      |  ,     is  the  simplest  known  are,  as 

COOH 

has  been  said  above,  half  alcohols  and  half  acids,  the 
alcoholic  character  being  imparted  to  them  by  the  pre- 
sence of  the  group  OH,  in  combination  with  unoxidized 
carbon,  and  the  acid  character  by  the  presence  of  the 
group  COOH.  All  that  has  been  said  in  regard  to  the 
alcoholic  group  OH  holds  good  in  regard  to  that  group 
in  these  diatomic  acids ;  and  all  that  has  been  said  in 
regard  to  the  carboxyl  group  holds  good  in  regard  to 
that  group  in  these  acids. 

That  the  above  formula  for  glycolic  acid  is  correct, 
follows  from  the  methods  of  its  preparation.  It  is  a  pro- 
duct of  the  partial  oxidation  of  ethylene  alcohol — 

CH^OH 

|  ,    one  of  the  primary  alcohol  groups  being  con- 

CH.OH 
verted  into  carboxyl.   The  fact  that  glycolic  acid  itself  by 

CO.OH 
further  oxidation  is  converted  into  oxalic  acid,      | 

CO.OH 

proves  also  that  the  group  CH..OH  is  present  in  it. 

It  is  further  obtained  by  treating  chlor-  or  bromacetic 
acid  with  silver  oxide,  thus : — 

CH2Br  CH..OH 

|  +       AgOH         ==          |  +     AgBr  . 

COOH  COOH 

Bromacetic  Glycolic  acid, 

acid. 

Lactic  or  oxypropionic  acid  is  the  succeeding  homo- 
logue  of  glycolic  or  oxyacetic  acid.  As  it  is  a  monosub- 
stitution  product  of  propionic  acid,  there  must  be  two 
varieties  possible  corresponding  to  a-  and  /3-chlorpropi- 
onic  acids.  One  of  these  would  have  the  formula — 
H  OH 


-C C— < 


H— C C— CO.OH  ,      and  the  other,  the  formula— 

i   i 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        203 

H    H 

HO—  C—  C—  CO.OH  .      Both  of  these  acids  are  known. 

I       I 
H    H 

The  first  is  ordinary  lactic  acid,  or  ethylidenelactic  acid. 
The  second  is  sarcolactic  acid,  or  ethylene  lactic  acid. 

Proofs.  —  The  proofs  of  these  formulas  are  the  follow- 
ing :  Sarcolactic  is  obtained  by  boiling  cyanhydrine  with 

CH2.OH 
alkalies.     Cvanhydrine,     |  ,  is  obtained  from  ethy- 

CH2.CN 


lene  chlorhydrine,     |  ,  by  treating  the  latter  with 

CH.2.C1 

potassium  cyanide.  Ethylene  chlorhydrine  is  obtained 
by  treating  ethylene  with  hypochlorous  acid.  Ethylene 

CH2 
is     ||        ,    as  has  been  shown.     Further,  sarcolactic  acid 

CH2 

contains  the  group  CH2OH,  for,  by  oxidation,  it  yields 
an  acid  containing  the  same  number  of  carbon  atoms. 
If  it  contains  the  group  CH2OH,  it  must  have  the  consti- 


tution  represented  by  the  formula    CH  ,  which 

\COOH 
is  that  above  given. 

The  only  other  possible  compound  of  this  composition 


must  have  the  formula     CH(OH)<^  .         Conse- 

XCOOH 

quently,  the  latter  is  the  formula  of  ordinary  lactic  acid. 
Such  a  compound  could  not,  by  oxidation,  yield  an  acid 
containing  the  same  number  of  carbon  atoms.  Ordinary 
lactic  acid  breaks  up  by  oxidation,  yielding  both  formic 
and  acetic  acids. 

The  remaining  diatomic  acids  of  the  series  have  not 
been  as  well  studied  as  the  few  which  have  here  been 
considered. 


204  CHEMICAL    COMPOUNDS. 

One  peculiarity  of  the  above  acids  should  be  noticed. 
They  are  monobasic  acids,  but  still  they  are  capable  of 
forming  anhydrides  by  losing  one  molecule  of  water  from 
one  of  their  own  molecules.  The  anhydrides  thus  formed 
differ  somewhat  from  the  ordinary  anhydrides  of  acids. 

/-°-\ 
Thus  we  have  glycolic  anhydride,    CH./  ;>  ,  formed 

XCCK 
by  abstracting  one  molecule  of  water  from  one  molecule 


of    glycolic   acid,      CH  ;      lactic    anhydride, 

\COOH 

/-°-\ 
CEL.CH7  >  ,      formed  from  one  molecule  of  ordi- 

\CO/ 

OH 
nary  lactic  acid,  CH,.CH< 

\COOH 

Bibasic   Acids,  CnH2n_z04  —  Oxalic   acid,  C2H204,   or 
COOH 

,    is  the  simplest  representative  of  these  acids 
COOH 

possible.  The  fact  that  it  is  bibasic,and  that  the  number 
of  groups  COOH  contained  in  a  compound  determines 
its  basicity,  leads  to  the  formula  given. 

The   second   member  of  this  series  is  malonic  acid, 


. 

CH2<  .      Of  each  of  these  acids  only  one  variety 

\COOH 
is  possible. 

The  third  member  is  succinic  acid,    C2H 

\COOH 

Of  this  there  must  be  two  varieties  corresponding  to  the 
two  lactic  acids,  or  the  two  series  of  mono-substitution 
products  of  propionic  acid.  For  succinic  acid  may  plainly 
be  considered  as  propionic  acid  in  which  a  hydrogen  atom 
has  been  replaced  by  a  carboxyl  group.  The  twro  &uc- 
cinic  acids  would  have  the  following  formulas  :  — 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        205 

H      CO.OH  H    H 

I  I  II 

1.   H— C—C— CO.OH  and  2.  CO.Ofl— C— C— CO.OH 

II  II 
H     H                                                 H    H 

The  second  formula  is  that  of  ordinary  succinic  acid, 
and  the  first  that  of  isosuccinic  acid. 

Proofs. — Ordinary  snccinie  acid   is   obtained  from   /3- 
eyanpropionic  acid,  the  constitution  of  which  we  know 
H   H 


be      CN— C— C— CO.OH  ;      and  from  ethylene  cya- 


to 


CH2.CN 

nide,     |  ,     which  Is  obtained  by  treating  ethylene 

CH,.CN 

bromide  with  potassium  cyanide. 

Isosuccinic  acid  is  obtained  from  a-cyanpropionic  acid, 
H    CN 

which  is     H— C— C— COOH  . 

A 

Fifth  Group. 
Triatomic  Alcohols  and  Acids. 

Glycerin. — Only  one  alcohol  is  well  known  which  con- 
tains  three   hydroxyl  groups.     This  is  glycerin.     Such 
alcohols  are  known  as  triatomic  alcohols.     The  formula 
CH,OH 

of  glycerin  is     CHOH      .      This  formula  is  very  pro- 

OET,pH 

bable,  because,  as  a  result  of  a  large  number  of  observa- 
tions of  carbon  compounds,  it  seems  to  be  a  general  fact 
that  one  carbon  atom  cannot  hold  in  combination  more 
than  one  hydroxyl  group.  If  this  is  true,  the  above  for- 
18 


206 


CHEMICAL    COMPOUNDS. 


mula  is  the  only  one  possible  for  glycerin.  But,  again, 
by  oxidation,  glycerin  yields  a  monobasic  acid  containing 
the  same  number  of  carbon  atoms;  and,  by  further  oxi- 
dation, apparently  a  bibasic  acid  also  containing  the  same 
number  of  carbon  atoms.  These  facts  would  show  that 
the  group  CH.,OH  occurs  twice  in  glycerin.  But  if  there 
are  two  groups  CH2OH  present  in  glycerin,  then  the  for- 
mula above  accepted  must  be  correct. 

Glyceric  Acid  is  obtained  by  partially  oxidizing  gly- 
cerin. As  the  acid  contains  the  same  number  of  carbon 
atoms  as  glycerin  contains,  it  is  assumed  that  the  oxi- 
dation consists  in  a  transformation  of  the  primary  alco- 
hol group  CH.OH  into  COOH;  hence,  the  formula  of 

CH8OH 

I 
glyceric  acid  is    CHOH 

COOH 
COOH 


According  to  this,  a  bibasit; 


acid  ,     CHOH   ,      ought   to  be  obtained    by  oxidizing 

COOH 

glyceric  acid,  just  as  this  bibasic  acid  is  obtained  by  oxi- 
dizing glycerin.  This  transformation  has  not  yet  been 
effected. 

Sixth  Group. 
Tetr atomic  Compounds. 

The  best  known  members  of  this  group  are  tartaric 
acid  and  citric  acid.  The  former  is  a  bibasic  acid,,  con- 
taining, in  addition  to  the  two  carboxyl  groups,  two 
alcoholic  hydroxyl  groups.  Jt  is  hence  a  bibasic 
tetratomic  acid.  It  is  dioxysuccinic  acid,  and  must 

CH.OH.COOH 

have  the  formula       |  It    is    obtained 

CH.OH.COOH 

from  dibromsuccinic  acid  by  treating  the  latter  with 
water,  thus : — 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        207 

CHBr.COOH  CH.OH.COOH 

|  -f  2H  0  =     |  -f-  2HBr  . 

CHBr.COOH  CH.OH.COOH 

Dibromsucciuic  acid.  Tavtaiic  acid 

Citric  acid  is  tribasic,  containing,  in  addition  to  its 
three  carboxyl  groups,  one  alcoholic  hydroxyl.  It  is 
hence  a  tribasic  tetratomic  acid. 


In  addition  to  the  groups  above  referred  to,  there  are 
pentatomic  and  hexatomic  compounds.  Of  the  former, 
there  is  only  one  representative  known.  Of  the  latter, 
however,  a  large  number  of  members  are  known.  Among 
these  are  the  different  varieties  of  sugars,  cellulose,  and 
starch;  and  the  acids  which  are  derived  from  them.  All 
that  is  positively  known  of  these  compounds  is  that  they 
contain  a  certain  number  of  hydroxyl  groups,  or  of 
hydroxyl  and  carboxyl  groups.  The  presence  of  the 
carboxyl  groups  is  detected  through  the  acid  properties 
of  the  substance.  If  the  substance  is  a  monobasic  acid, 
one  carboxyl  group  is  assumed  as  being  present  in  it ;  if 
it  is  a  bibasic  acid,  two  carboxyl  groups  are  assumed  as 
being  present  in  it,  etc.  The  number  of  hydroxyl  groups 
present  is  determined  by  allowing  acetyl  chloride  or 
acetic  anhydride  to  act  upon  the  compound.  If  the  latter 
contains  only  one  hydroxyl,  it  will  take  up  only  one 
acetyl  group,  C2HS0  ;  if  it  contains  two  hydroxyl  groups, 
it  will  take  up  two  acetyl  groups,  etc. 

Seventh  Group. 
Cyanogen  Compounds. 

In  speaking  of  the  group  CN  as  a  substituting  group, 
the  proofs  for  the  formula  — C^N  for  this  group  were 
given  (ante,  p.  152).  Now  this  same  group  is  obtained 
from  the  compounds  known  as  cyanides,  and  hence  the 
cyanides  have  an  analogous  constitution.  Cyanogen 

C~N 
itself  has  the  formula  C2N2  or       |  .       The  simplest 

C=N 
compound  of  cyanogen  is  hydrocyanic  acid,  which  con- 


208  CHEMICAL    COMPOUNDS. 

sists  of  the  group  — C— N  combined  with  hydrogen,  viz., 
H— C=N. 

The  Itydrog^n  atom  of  this  acid  may  be  replaced  by  a 
variety  of  groups  or  other  elements,  as,  for  instance, 
OH,  SH,  NH.2,  etc.  Then  a  large  number  of  derivatives 
are  obtained  which  have  a  constitution  similar  to  that  of 
the  acid.  Thus  we  have  cyanic  acid,  HO — C:^N ;  sul- 
phocyanic  acid,  HS — C— N;  cyanamide,  H2N — C^N, 
etc. 

It  has  already  been  shown  that  there  are  compounds 
containing  the  group  CE=N —  called  carbylamines,  which 
are  isomeric  with  the  cyanides  of  hydrocarbon  residues, 
and  the  proofs  for  the  formula  Ci^N —  have  also  been 
given  (see  ante,  p.  153). 

Mustard,  Oils. — Sulphocyanic  acid,  HS — C— N,  like 
other  acids,  yields  salts  and  ethers  by  exchanging  its 
hydrogen  for  metals  or  hydrocarbon  residues.  We  have 
potassium  sulphocynnate,  KS — C=N;  methyl  sulpho- 
cyanate,  CH3 — S — C==N,  etc.  Running  parallel  to  the 
ethers  of  sulphocyanic  acid  is  a  series  of  compounds 
known  as  mustard  oils.  These  have  the  same  composi- 
tion as  the  above  ethers,  but  entirely  different  properties 
and  constitution. 

The  simplest  representative  of  this  series  is  methyl 
mustard  oil,  which  has  the  constitution  expressed  by  the 
formula  S=C=N — CH3.  A  number  of  corresponding 
compounds  are  known,  one  of  which  is  allyl  mustard  oil, 
S=C=N — C3H5.  This  is  the  oil  obtained  from  black 
mustard  seed. 

The  proofs  of  the  constitution  assigned  to  the  mustard 
oils  are  as  follows  : — 

Ethyl  mustard  oil  is  formed  by  a  somewhat  circuitous 
method.  When  carbon  bisulphide,  CS^,  is  brought  in 

/CSH5 
contact  with  ethylamine,      N — H        ,      the   ethylarnine 

\H 
salt  of  ethyl  sulphocarbamic  acid  is  formed,  thus: — 

CS2     +     2(NH,.C2H5)     =     CS< 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        209 

I5y  appropriate  reactions,  this  salt  is  split  up  into 
ethylamine,  hydrogen  sulphide,  and  ethyl  mustard  oil. 
The  decomposition  can  be  best  interpreted  as  follows: — 


--/NJHJ.C.H. 

^J  O  ,      ,  ei   rr     I  TVT  TT 


H|.|NH..C,H5   ' 

Hence  the  resulting  mustard  oil  retains  an  atom  of 
sulphur,  combined  by  means  of  two  affinities  with  carbon, 
and  the  residue  of  ethylamine,  =N.C.JH5,  being  bivalent, 
would  naturally  be  held  b}'  the  two  remaining  affinities 
of  the  carbon.  But,  if  we  examine  the  products  of  de- 
composition of  ethyl  mustard  oil,  we  are  also  led  to  the 
formula  above  given.  With  water  or  hydrochloric  acid 
it  yields  ethylamine,  carbon  dioxide,  and  hydrogen 
sulphide;  with  nascent  hydrogen  it  yields  ethylamine 
formylsulphaldehyde  and  hydrogen  sulphide.  The  pro- 
duction of  ethylamine  indicates  clearly  that,  in  the  mus- 
tard oil,  the  ethyl  group  is  in  combination  with  the 
nitrogen  atom;  and  the  production  of  formylsulphalde- 
hyde, which  differs  from  formic  aldehyde,  H.COH,  only 
in  containing  sulphur  in  the  place  of  oxygen,  also  indi- 
cates that  in  ethyl  mustard  oil  the  sulphur  atom  is  in 
combination  with  carbon.  These  results  are  embodied  in 
the  formula  accepted  for  the  mustard  oil. 

The  ether  of  sulphocyanic  acid,  which  is  isomeric  with 
ethyl  mustard  oil,  conducts  itself  towards  reagents  in  an 
entirely  different  manner.  It  never  yields  ethylamine, 
but  always  yields  a  compound  in  which  the  ethyl  group 
is  in  combination  with  sulphur,  as  ethylsulphide  or  ethyl- 
sulphurous  acid ;  while  the  nitrogen'  is  split  off  in  com- 
bination with  hydrogen  alone,  or  with  carbon,  hydrogen, 
and  oxygen. 

Eighth  Group. 
Derivatives  of  Carbonic  Acid. 

The  salts  of  carbonic  acid  have  the  general  formula 
M2COS.  They  are  derived  from  a  bibasic  acid,  H2C(X. 
This  acid  being  bibasic  contains  two  hydroxyl  groups, 

OH 

and  hence  we  are  led  to  the  formula       C0(  for 

18* 


210  CHEMICAL    COMPOUNDS. 

carbonic  acid.  No  such  acid  is  known,  however,  If  we 
attempt  to  prepare  it  from  its  salts,  we  always  get  the 
compound  C02,  which  may  justly  be  considered  as  the 
anhydride  of  the  true  carbonic  acid.  It  has  already  been 
stated,  that  it  appears  to  be  a  general  truth,  that  one 
carbon  atom  cannot  hold  in  combination  more  than  one 
hydroxyl  group.  This  breaking  up  of  carbonic  acid  into 
water  and  the  anhydride  is  in  harmony  with  the  general 
truth.  Whether  the  acid  is  formed  or  not  when  the 
anhydride  is  conducted  into  water  is  not  yet  decided. 

Though  we  are  not  acquainted  with  carbonic  acid,  we 
are  acquainted  with  a  very  large  number  of  its  derivatives. 
These  are  obtained,  1,  by  replacing  the  hydrogen  of  the 
acid  by  elements  or  groups;  2,  by  replacing  one  or  both 
of  the  hydroxyl  groups  by  groups  or  elements;  3,  by 
replacing  the  oxygen  by  sulphur.  Thus  we  obtain  first 
a  series  of  salts  and  ethers  ;  then  compounds,  such  as 


carbonyl  chloride,     C=O    ,     carbon  sulphoxide, 
\C1 

^s  ,s 

C^       ,      carbon  bisulphide,     CZ      ;     and  finally  snch 
^O  ^S 

SH 


com 


pounds  as  sulphocarbonic  acid,       CS<^          ,      xan- 


ihogenic  acid,         CS<^  ,  etc. 


Among  the   most  important  derivatives   of  carbonic 
acid  is  the  amide  urea  or  carbamide,  which  has  the  con- 

NH, 
stitution  expressed  by  the  formula     CO 


The  proofs  of  this  formula  are  as  follows  :  — 
It  is  formed  by  the  action  of  carbonyl  chloride  upon 
ammonia,  thus  :  — 


COC12     -f     2NH3     =    C0<  +     2HC1  . 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        211 

Also  by  the  action  of  ammonia  upon  ethylcarbonate, 
thus: — 

OC2H6 

C0<  +    2NH3    =     C0 

N)C2H5 

The  latter  is  a  general  reaction  employed  for  the  pro- 
duction of  acid  amides  from  the  ethers. 

Urea  has  the  power  of  combining  with  bases,  acids, 
and  salts,  and  of  forming  with  them  crystallizing  com- 
pounds. Instead  of  the  ammonia  residue  NH.^,  further, 
it  may  contain  residues  of  the  amine  bases,  as  NH.CHS, 
NH.C2H5,  etc.  Or,  again,  one  or  more  of  the  hydrogen 
atoms  of  urea  may  be  replaced  by  acid  residues,  such  as 
C2H30,  C7H5O,  etc. 


A  large  number  of  compounds  are  allied  to  and  derived 
from  uric  acid.  They  have  frequently  been  the  subjects 
of  exhaustive  investigations,  but,  up  to  the  present,  no 
formula  has  been  proposed  for  uric  acid  which  is  in  every 
respect  satisfactory.  It  is  a  weak  bibasic  acid,  but  it 
does  not  contain  two  carboxyl  groups,  for  its  formula  is 
C5N4H403,  while  a  compound  which  contains  two  carb- 
oxyl groups  must  contain  four  atoms  of  oxygen.  The 
presence  of  the  group  — C^rN  seems  to  be  pretty  clearly 
indicated  in  the  acid,  for  it  yields,  with  great  ease,  pro- 
ducts which  certainly  contain  this  group.  The  amide 
group  NH.2  is  also  probably  present  in  it,  for,  when  heated 
with  hydriodic  acid,  it  vields,  among  other  products,  gly- 

/NH, 
cocol  or  amido-acetic  acid,     CH  { 

XCOOH 


BENZENE  DERIVATIVES.     (AROMATIC  BODIES.) 

A  large  class  of  compounds  exists  which  possess  the 
property  in  common  that,  when  decomposed  in  a  number 
of  ways,  they  yield  benzene  as  one  of  the  products. 
Benzene  itseli'  has  the  formula  C6H6.  Just  as"  the  mem- 
bers of  this  class  of  compounds  yield  benzene  as  a  de- 
composition product,  so,  also,  they  may  all  be  built  up 


212  CHEMICAL    COMPOUNDS. 

from  benzene  by  the  introduction  of  a  variety  of  groups 
or  elements  in  the  place  of  hydrogen.  All  these  com- 
pounds bear  a  similar  relation  to  benzene  to  that  which 
the  fatty  bodies  bear  to  marsh-gas.  In  studying  the 
aromatic  bodies,  then,  it  is  plainly  our  first  duty  to  de- 
termine the  constitution  of  benzene  itself,  as  the  consti- 
tution of  the  derivatives  cannot  be  understood  until  this 
determination  is  made. 

Constitution  of  Benzene. — The  great  stability  of  ben- 
zene indicates  that  it  is  a  saturated  compound.  Now,  if 
the  carbon  atoms  contained  in  it  are  quadrivalent,  the 
simplest  hypothesis  which  can  be  formed  concerning 
its  constitution  would  be  indicated  in  the  following 
formula : — 

H 
C 


CH 
HC        CH 

V          •';  •' 

H 

According  to  this,  the  molecule  of  benzene  consists  of 
a  closed  chain  of  carbon  atoms,  each  united,  on  the  one 
hand,  by  two  affinities  with  another  carbon  atom ;  on  the 
other  hand,  by  one  affinity  with  a  second  carbon  atom. 

This  formula,  which  was  originally  proposed  by  Kekule, 
accounts  satisfactorily  for  nearly  all  the  facts  known 
concerning  aromatic  bodies.  These  facts  are  mainly  the 
following: — 

1.  Of  the  substitution  products  of  benzene  which  con- 
tain one  substituting  group,  only  one  variety  is  known. 

2.  Of  the  substitution  products  of  benzene  which  con- 
tain two  substituting  groups,  three  varieties  have  been 
observed,  and  only  three. 

3.  Of  the  substitution  products  of  benzene  which  con- 
tain three  substituting  groups,  more  than  three  varieties 
have  been,  observed.  • 

If  all  the  hydrogen  atoms  in  benzene  play  exactly  the 
same  parts,  then  the  first  fact  mentioned  would  follow  as 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        213 

a  matter  of  course.     In  the  above  formula,  all  the  hydro- 

fen  atoms   are  represented  as  playing  the  same  parts. 
]ach  one  is  situated  exactly  like  all  the  others  with  refe- 
rence to  the  whole  molecule. 

A  great  many  efforts  have  been  made  to  obtain  iso- 
meric  mono-substitution  products  of  benzene,  but  they 
have  all  been  unsuccessful. 

Again,  if  we  examine  the  above  formula  carefully,  we 
find  that  there  are  three  and  only  three  pairs  of  hydrogen 
atoms  in  it  which  differ  from  each  other  in  the  positions 
of  their  individual  atoms.  Numbering  these  hydrogen 
atoms  as  follows  : — 

1 

H 
C 

/\ 
6  HC        CH  2 

II        I 
5  HC        CHS 

\> 

C 

H 
4 

we  can  distinguish  the  following  pairs:  1.2,  1.3,  1.4,  1.5, 
1.6  ;  or,  beginning  with  2,  we  would  also  have  five  pairs  ; 
but,  as  all  the  hydrogen  atoms  of  benzene  play  exactly  the 
same  parts,  it  is  plainly  immaterial  with  which  one  we 
begin,  the  resulting  pairs  will  be  identical.  Thus,  1.2  is 
identical  with  2.3,  3.4,  4.5,  and  5.6 ;  1.3  is  identical  with 
2.4,  3.5,  4.6,  and  5.1;  1.4  is  identical  with  2.5,  3.6,4.1, 
and  5.2;  etc.  But,  further,  1.2  is  also  identical  with  1.6, 
and  1.3  with  1.5.  Hence,  of  the  five  original  pairs  we 
have  only  three  left.  These  are  1.2,  1.3,  and  1.4.  They 
are  the  only  ones  that  differ  from  each  other  essentially 
in  the  benzene  formula  of  Kekule.  If,  then,  substitution 
products,  containing  two  substituting  groups,  are  obtained 
from  benzene,  they  have  one  of  the  three  following  for- 
.mulas,  in  which  X  represents  a  monovalent  substituting 
group  or  element : — 


214  CHEMICAL    COMPOUNDS. 


ceo 
/  \  /  \  /  ,\ 

HO        CX  HC        CH  HC       CH 

1-          II         I       ;      2.          ||         |       ;      3.        ||         |      . 
HC        CH  HC       CX  HC       CH 

\S  \  ^  V> 

c  c  c 

H  H  X 

As  was  staterl  above,  only  three  varieties  of  bisubsti- 
tution  products  of  benzene  have  ever  been  observed.  So 
that  here,  again,  we  have  perfect  harmony  between  facts 
and  the  hypothesis. 

No  one  claims  that  the  benzene  formula  of  Kekuld 
represents  the  actual  arrangement  of  the  atoms  in  space. 
It  undoubtedly  represents  certain  truths,  however.  It 
represents  that  in  the  molecule  of  benzene,  the  hydrogen 
atoms  are  arranged  symmetrically,  and  that  all  the  parts 
of  the  molecule  are  symmetrically  arranged.  We  do  not 
know  positively  that  there  is  such  symmetry  in  the  ben- 
zene molecule,  for  we  know  nothing  of  molecules  them- 
selves, but,  from  all  the  facts  known  to  us,  it  seems  fair 
to  conclude  that  this  symmetry  of  the  different  parts  is 
characteristic  of  the  benzene  molecule. 

Substitution  Products  of  Benzene. — Of  mono-substi- 
tution products  we  have  only  one  variety.  We  have  only 
one  monochlorbenzene,  C6H5C1 ;  only  one  oxybenzene,  or 
phenole,  CfiH..OH;  only  one  benzole  acid,  C6H5.COOH; 
only  one  toluene,  C6H5.CH3,  etc.  etc.  The  constitution 
of  most  of  these  derivatives  is  very  simple.  There  is  a 
peculiarity,  however,  connected  with  those  which  are 
formed  by  replacing  one  hydrogen  atom  of  benzene  with 
a  hydrocarbon  residue.  The  simplest  compound  formed 
in  this  way  is  toluene,  which  consists  of  benzene  in  which 
a  hydrogen  atom  has  been  replaced  by  the  methane  resi- 
due CH3 ;  if,  instead  of  the  residue  CH,,  we  introduce 
C2H.,  we  obtain  ethylbenzene,  C6H5.C2H5,  which  is  plainly 
an  homologue  of  toluene;  so,  also,  the  residues  C3EJ.,  C4H,, 
C.Hn,  etc.,  may  £e  employed,  and  thus  we  obtain  an 
homologous  series  of  aromatic  hydrocarbons,  all  of  which 
are  mono-substitution  products  of  benzene.  These  may, 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        215 

further,  all  be  regarded  as  substitution  products  of  the 
hydrocarbons  of  the  methane  series.  Accordingly,  of 
toluene  and  ethylbenzene,  which  are  mono-substitution 
products  of  methane  and  ethane  respectively,  only  one 
variety  each  is  possible  ;  while  of  the  next  homologue,  or 
propylbenzene,  CaH5.C3H.,  two  varieties  are  possible,  cor- 
responding to  the  a-  and  |3-monosubstitution  products  of 
pro pyl,  or  to  the  prop}7!  and  isopropyl  compounds  (which 
see).  The  main  members  of  the  series  of  hydrocarbons 
thus  referred  to  are  : — 

Benzene,  C6H6. 

Toluene  or  methylbenzene,  C7H8  or  C(iH5.CH3. 

Ethylbenzene,  C8H10  or  C6H5.C2H5. 

Propylbenzene,  C9H]a  or  CfLCJBL. 

Butylbenzene,  C10HI4  or  C6H5.C4H9. 

Amylbenzene,  CnH15or  C6H5.C5Hn. 

Of  these  hydrocarbons,  two  kinds  of  mono-substitution 
products  are  possible,  viz.,  those  in  which  the  substi- 
tuting group  or  element  is  situated  in  the  benzene  nucleus, 
and  those  in  which  the  substituting  group  or  element  is 
situated  in  the  other  residue.  These  other  residues, 
however  they  may  be  constituted,  are  known  as  lateral 
chains.  It  is  plain  that  substitution  products  of  the  latter 
kind  correspond  closely  to  those  of  the  hydrocarbons  of 
the  methane  series,  and  hence  they  need  no  special  con- 
sideration here.  If  a  substituting  group  or  element 
enter  into  the  benzene  nucleus  of  any  of  these  hydro- 
carbons, of  course  we  have  no  longer  to  deal  with  mono- 
substitution  products  of  benzene. 

Bisnbstitution  Products. — The  three  classes  of  bi- 
derivatives  of  benzene  which  we  have  above  recognized 
as  possible,  have  been  designated  respectively  as  ortho, 
meta,  and  para  compounds,  or,  by  others,  as  1.2.  1.3,  and 
1.4  compounds.  The  former  expressions  are  to  be  pre- 
ferred, for  the}7  are  independent  of  any  hypothesis  con- 
cerning the  positions  of  the  substituting  groups.  It  is 
usual  to  consider  the  expressions  ortho  and  1.2,  meta 
and  1.3,  para  and  1.4,  as  identical,  but  this  implies  that 
the  following  formulas  have  been  proved,  while  they  have 
not  been  : — 


216  CHEMICAL    COMPOUNDS. 

XXX 

c  c  c 

/\  /\  /\ 

HC       CX  HC       CH    .         HC       CH 


HC 


I      ;          II       i      ;         II       I      - 

CH  HC       CX  HC       CH 


c  c  c 

H  H  X 

Ortho-compound.  Meta-compound.  Para-compound. 

What  we  really  know  is  that  there  are  three  classes  of 
these  bi-substitution  products,  and  that  the  members  of 
any  one  of  these  classes  can  be  converted  into  each  other, 
thus  showing  that  they  are  allied.  There  are  three  com- 
pounds, each  representing  one  of  the  three  classes  to 
which  all  other  bi-substitution  products  are  referred,  if 
possible.  If  the  constitution  of  any  such  product  is 
unknown,  it  is  only  necessary  to  convert  it  into  one  of 
the  three  compounds,  when  the  series  to  which  it  belongs 
is  assumed  to  be  known.  The  three  compounds  are  the 

/COOH 
isomeric,  bicarbonic  acids  of  benzene,      C6H  /  , 

\COOH 

viz.,  phthalic,  isophthalic,  and  terephthalic  acids.  All  bi- 
substitution  products  which  can  be  converted  into  phthalic 
acid  are  known  as  ortho-compounds  ;  all  that  can  be  con- 
verted into  isophthalic  acid  are  known  as  meta-compounds  ; 
and  all  that  can  be  converted  into  terephthalic  acid  are 
known  as  para-coin  pounds. 

The  conversion  into  these  acids  need  not  always  be 
direct.  If  it  be  possible  to  convert  a  compound  into 
another  which,  in  its  turn,  can  be  converted  into  one  of 
the  above  acids,  the  same  conclusion  is  drawn  as  in  the 
case  of  a  direct  conversion.  Of  course,  the  accuracy  of 
the  conclusions  drawn  with  reference  to  the  constitution 
of  bi-substitution  products  depends  upon  the  trustworthi- 
ness of  the  reactions  employed  in  effecting  the  conver- 
sions. Some  reactions  employed  for  this  purpose  have 
been  found  to  give  inaccurate  results  ;  jthat  is  to  say,  the 
products  resulting  from  an  application  of  these  reactions 
belong  to  different  series  from  those  to  which  the  original 
compounds  belonged.  It  is  very  probable  that  some 


CONSTITUTION    OP    CHEMICAL    COMPOUNDS.        217 

compounds  now  classified  with  one  series  in  consequence 
of  some  conversion,  may  be  found,  by  future  investiga- 
tions, to  belong  to  a  different  series. 

The  formulas  given  above  as  representing  the  relative 
positions  of  the  substituting  groups  in  ortho-,  meta-,  and 
para-compounds  are  based  upon  the  following  facts  :— 

It  will  be  shown  that  naphthalene  (which  see;  probably 
has  the  formula  — 

H     H 
C—  C 

HC  CH 

\        / 

c=e 

/     \ 

HC1.     2.CH 

\        / 

C—  C 
H     H 

By  oxidation,  naphthalene  yields  phthalic  acid.  It  seems 
probable,  therefore,  that  the  carboxyl  groups  in  the  acid 
have  the  same  relative  position  as  that  of  the  groups 
numbered  1  and  2  in  this  formula.  Consequently,  ortho- 
compounds,  or  those  which  can  be  converted  into  phthalic 
acid,  have  their  substituting  groups  in  the  positions  1.2 
in  the  benzene  nucleus  ;  or,  what  is  the  same  thing,  the 
substituting  groups  in  ortho  compounds  are  combined 
with  adjacent  carbon  atoms. 

It  will  also  be  shown  that  mesitylene  (which  see)  prob- 
ably has  the  formula  — 


HC       CH 

II         I 
CH3.C        C.CH3 

\S     ' 
C 
H 

By  partially  oxidizing  this   hydrocarbon,  an   acid    is 
obtained  of  the  formula  — 
19 


218  CHEMICAL    COMPOUNDS. 

COOH 

C 

/\ 
HC       CH 

II         I 
CH.C        C.CH3 

V 

H 

When  this  acid  is  heated  under  proper  conditions 
carbon  dioxide  is  given  off  and  a  hydrocarbon  is  obtained 
of  the  formula — 

H 
C 

x\ 

HC       CH 

!!        I 
CH.C         C.CH, 


H 

Lastly,  when  this  hydrocarbon  is  oxidized,  both  the 
groups  CH3  are  converted  into  COOH,  and  the  resulting 
acid  is  isophthalic.  Hence,  if  the  formula  of  mesitylene 
is  correct,  that  of  isophthalic  acid  is  also  correct. 

By  exclusion,  terephthalic  acid  becomes  a  1.4  com- 
pound, and,  consequently,  all  para-compounds  are  1.4 
compounds. 

It  must  be  confessed  that  these  proofs  are  not  strong 
enough  to  command  universal  respect  among  chemists. 
As  the  expressions  1.2,  1.3,  and  1.4  are  in  common  usage, 
it  is  well,  however,  to  know  the  grounds  upon  which 
their  use  is  based.  Some  of  the  principal  bi-substitution 
products  of  benzene  are  given  in  the  following  table, 
which  shows  also  to  which  series  the  compounds  belong: — 

Ortho.  Meta.  Para 

Phthalic  acid,  .    Isophthalic  acid,          Terephthalic  acid, 

Orthoxylene,  Isoxylene,  Xylene, 

Salicylic  acid,  Oxybenzoic  acid,         Paroxybenzoic  acid, 

Pyrocatechin,  Resorcin,  Hydroqninone, 

Orthodinitrobenzene,    Metadinitrobenzen'e,  Paradinitrobenzene, 
Orthobibrotii benzene.  Mctabibrombenzene.  Parabibrombenzene. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.       219 

It  is  well  to  note  here  that  whether  both  substituting 
groups  be  the  same  or  not,  only  three  kinds  of  products 
can  be  obtained. 

Tri-substitution  Products. — The  most  important  of  the 
tri-substitution  products  of  benzene  is  rnesitylene.  The 
formula  of  this  hydrocarbon  is  CyH^.  By  oxidation  it 
yields,  according  to  the  energy  of  the  reaction,  three 
different  products.  The  first,  mesitylenic  acid — 
C8H9.COOH,  is  monobasic;  the  second,  uvitic  acid — 
C7Hg.(COOH)11  is  bibasic  ;  and  the  third,  trimesinic  acid, 
C€HS.(OOOH)3,  is  tribasic.  All  of  these  acids,  when 
heated  with  lime,  yield  either  benzene  itself  or  derivatives 
of  benzene.  Hence,  it  is  concluded  that  mesitylene  is 
benzene  in  which  three  hydrogen  atoms  are  replaced  by 
the  residue  CH3  thus,  Cf6H3(C H3)3.  By  oxidation  each 
one  of  these  groups  in  turn  is  converted  into  carboxyl, 
yielding  thus  the  three  acids  above  mentioned.  It  still 
remains,  however,  to  decide  what  the  positions  of  these 
three  substituting  groups  in  the  benzene  nucleus  are. 
•  The  following  method  of  consideration  leads  to  the 
formula  for  mesitylene  given  on  page  2 It : — 

When  acetone  "is  treated  with  concentrated  sulphuric 
acid,  water  is  abstracted  and  the  residues  of  three  mole- 
cules unite  to  form  mesitylene.  It  seems  to  be  fair  to 
assume  that  the  three  residues  are  constituted  exactly 
the  same,  as  they  are  formed  under  exactly  the  same 
conditions,  from  the  same  compound.  If  they  are  the 
same,  they  must  each  be  C3H4.  Three  such  residues 
might  be  formed  from  acetone,  thus  : — 

Acetone  is  CH3 — CO — CH3;  three  molecules  may  be 
arranged — 

CH3 


H 
CH 


o 

c 

^|5~    XCHffi 

n 

I13 

1,-c       o 

i 

0 

C—  CII3 

/ 

£ 

H/ 

220  CHEMICAL    COMPOUNDS. 

If  water  is  abstracted  in  the  manner  indicated  by  the 
lines,  we  have  left  three  residues,  08H4,  and,  if  these 
unite,  they  would  form  a  compound  of  the  constitution 
represented  by  the  following  formula: — 

CH, 


HC      CH 
CH,— C      C— CH3 

'  V 

H 

This  is  the  formula  accepted  for  mcsitylene;  and  from 
this  we  conclude,  as  above  seen,  that  meta-compounds 
have  their  substituting  groups  in  the  positions  1.3. 

If  this  formula  is  carefully  examined,  it  will  be  seen 
that  each  one  of  the  three  hydrogen  atoms  remaining  in 
the  benzene-nucleus  occupies  a  similar  position  to  that 
occupied  by  the  other  two.  Accordingly,  if  this  formula 
is  correct,  we  should  expect  to  find  that,  by  the  intro- 
duction of  one  substituting  group  into  mesitylene,  only 
one  product  would  be  formed.  This  has  actually  been 
found  to  be  true. 

Besides  mesitylene,  there  are  many  tri-substitution 
products  of  benzene  known,  containing  such  elements  as 
01,  Br,  I,  and  such  groups  as  N02,  NH.,,  SO.,OH,  etc. 
The  principle,  according  to  which  the  position  of  the 
substituting  groups  in  these  compounds  is  determined,  is 
this :  One  of  the  groups  is  split  off,  and  the  constitution 
of  the  resulting  bi-substitution  product  is  determined  as 
above;  then  from  the  original  compound  some  other 
group  is  split  off,  and  the  constitution  of  the  bi-substitu- 
tion product  resulting  in  this  case  also  determined.  We 
are  thus  able  to  judge  of  the  positions  of  the  three  groups 
with  reference  to  each  other.  There  are  not  many  com- 
pounds, however,  which  can  be  subjected  to  this  kind  of 
examination  with  satisfactory  results,  so  that  the  con- 
stitution of  these  tri-clerivatives  is  not  really  as  well 
known  as  that  of  the  bi-derivatives. 

Those  derivatives   of  benzene  which   contain   four  or 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        221 

five  substituting  elements  or  groups  have  not  been  very 
full}7  investigated. 

Also  only  a  few  hexa-derivatives  are  known,  the  most 
important  of  which  is  mellitic  acid,  C(i(OOOH)6,  or 
benzene,  in  which  all  six  hydrogen  atoms  are  replaced  by 
carboxyl  groups. 

Peculiar  Benzene  Derivatives.  —  Among  benzene 
derivatives  we  find  three  classes  which  are  not 
represented  among  the  fatty  bodies,  and  hence  they 
require  some  attention  here.  These  are  the  jjhenoles, 
quinones,  and  azo-bodies. 

Phenoles. — Phenoles  are  the  oxy-derivatives  of  ben- 
zene and  its  homologues,  formed  by  the  introduction  of 
hydroxyl  into  the  place  of  hydrogen  in  the  benzene 
nucleus.  The  corresponding  compounds  of  the  hydro- 
carbons of  the  methane  series  are  all  alcohols,  either 
primary,  secondary,  or  tertiary.  The  phenoles  are, 
however,  not  alcohols  in  the  sense  in  which  that  term 
has  been  used  up  to  the  present.  By  oxidation  they 
yield  neither  aldehydes,  acids,  nor  ketones. 

The  presence  of  hydroxyl  in  phenoles  can  be  proved 
in  the  same  way  that  it  was  proved  for  other  bodies 
containing  hydroxyl. 

There  are  monatomic  phenoles,  containing  only  one 
hydroxyl;  biatomic  phenoles,  containing  two  hydroxyls; 
triatomic  phenoles,  containing  three  hydroxyls,  etc. 

Quinones. — The  quinones  are  derived  from  benzene 
and  its  homologues  by  the  introduction  of  two  atoms  of 
oxygen  in  the  place  of  two  hydrogen  atoms  in  the  ben- 
zene-nucleus. Thus  the  simplest  quinone  has  the  formula 
C6H4Ok.  The  two  oxygen  atoms  are  supposed  to  form  a 
bivalent  group,  — 0 — O — ,  by  combining  with  each  other 
by  means  of  one  of  their  affinities  each.  Most  quinones 
are  derived  from  para-compounds  by  oxidation,  as  from 
hydroquinone,  and  hence  it  is  concluded  that  the  hydrogen 
atoms  replaced  by  the  bivalent  group  — O — 0 —  in  the 
formation  of  quinones  usually  occupy  the  para-position 
with  reference  to  each  other.  Accordingly,  if  the  para- 
position  is  1.4,  the  formula  of  ordinary  quinone  is — 

19* 


222  CHEMICAL    COMPOUNDS. 

C 

HC  0    CH 

|     IS     nUin      =      C°H<°" 

Y     •     '  ;f|p 

All  quinones  are  supposed  to  be  similarly  constituted, 
though  it  is  still  a  question  whether  all  quinones  are 
para-compounds.  Certain  experiments  seem  to  indicate 
that  there  are  quinones  which  belong  to  the  meta-series. 

Azo-  and  Diazo-Bodies. — These  bodies,  as  their  names 
imply,  are  nitrogen  derivatives.  They  are  derived  from 
benzene  and  its  homologues  by  the  replacement  of  hydro- 
gen by  nitrogen.  We  shall  consider  those  which  are 
derived  from  benzene,  as  the  others  are  very  closely 
related  to  these,  and  will  be  understood  if  these  are. 
The  diazo-derivatives  of  benzene  are  obtained  from  the 
salts  of  anilin  or  amidobenzene,  C6H5.NHV,  by  the  action 
of  nitrous  acid.  Thus  anilin  nitrate,  C6H5.NH2.HN03, 
yields  diazobenzene  nitrate  ;  anilin  sulphate, 
(C6H..NH2)a.H2S04,  yields  diazobenzene  sulphate,  etc. 

If  we  consider  simply  the  empirical  formulas  of  the 
salts  of  diazobenzene  thus  obtained,  we  shall  find  that 
they  differ  from  the  anilin  salts  in  containing  C6H4N2  in 
the  place  of  C6H.NH2.  The  salts  consist  of  the  acids 
plus  this  group.  Thus  the  nitrate  is  C6H4N2.HNO3;  the 
sulphate  is  C6H4N2.H2S04,  etc.  These  formulas  are  not 
supposed,  however,  to  represent  the  constitution  of  the 
salts.  If  the  group  C6H4N2  actually  existed  in  these 
diazo-bodies,  it  is  plain  that  they  would  be  bi-substitution 
products,  that  is  to  say,  two  hydrogen  atoms  of  benzene 
would  be  replaced  by  two  nitrogen  atoms.  It  was  at 
first  supposed  that  each  of  these  nitrogen  atoms  played 
the  part  of  a  monovalent  element,  and  the  diazo-com- 
pounds  were  looked  upon  as  analogous  to  bichlorbenzene, 
binitrobenzene,  etc.,  thus: — 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        223 


N 
C 

HC         CN 


Cl 
C 


HC 


CH 


HC 

=1 


CCl 

I 

CH 


C 
H 

Diazobenzene. 


H 

Bichlorbenzene. 


It  was  soon  found,  however,  that,  when  the  cliazo- 
bodies  were  decomposed,  they  almost  always  yielded 
derivatives  of  benzene  in  which  the  group  C6H5  was 
undoubtedly  present.  Thus  the  following  decomposi- 
tions of  diazobenzene  sulphate  yield,  in  each  case,  a 
derivative  containing  C6H5: — 

When  boiled  with  alcohol,  the  products  are  benzene, 
nitrogen,  and  sulphuric  acid — 


C6H5.X2.HS04 


H        H 


yield 


H 


H 


When   boiled  with  water,  the  products  are   phenole, 
nitrogen,  and  sulphuric  acid — 


C6H,.X2.HS04 
OH        H 


yield 


OH 


HS04 
H 


When  treated  with  hydriodic  acid,  the  products  are 
iodbenzene,  nitrogen,  and  sulphuric  acid — 


C6H5.X2.HS04 
I       H 


yield 


X. 


HS04 
H 


Other  reactions  indicate  as  well  that  the  group  C6H.  is 
present  in  the  diazo-compounds.  But,  if  this  group  is 
present,  the  two  nitrogen  atoms  must  form  a  monovalent 
group,  or,  at  all  events,  they  must  be  so  combined  that 
they  can  take  the  place  of  one  hydrogen  atom.  Xow,  if 
two  nitrogen  atoms  which  have  the  same  valence  be 
combined,  they  must  either  form  a  neutral  group  with 
all  its  affinities  satisfied,  or  a-  group  which  is  at  least 
bivalent.  Such  a  bivalent  group  would  be  formed,  for 


224  CHEMICAL    COMPOUNDS. 

instance,  if  two  nitrogen  atoms  were  to  be  united  by 
means  of  two  affinities  each,  thus,  — N=N — .  If  this 
group  should  replace  one  hydrogen  atom  of  benzene,  the 
constitution  of  the  resulting  compound  would  be 
C6H5 — N— N — .  iSuch  a  compound  would  be  unsaturated. 
No  compound  of  the  formula  CSH5N2  has  been  obtained, 
but  all  the  derivatives  of  diazobenzene  can  be  explained 
on  the  supposition  that  they  are  derived  from  the  com- 
pound C6H5 — N=N — .  Griess,  who  discovered  the 
diazo-  and  azo-compounds,  and  has  given  a  great  deal  of 
time  to  their  study,  has  described  a  body  of  the  formula 
C6H4N2,  which  he  calls  diazobenzene.  If  this  body  really 
exists,  the  conclusion  above  drawn  concerning  the  pre- 
sence of  the  group  CHH5  in  diazo-bodies  would  be  weak- 
ened;  but  there  seems  to  be  just  cause  to  doubt  its 
existence. 

Accepting  the  group  C6H5 — N=N —  as  the  foundation 
of  the  diazo-compounds,  these  may  be  formulated  as 
follows : — 

C6H5 — N=N — Br,  diazobenzene  bromide, 

C6H5 — N=N — NO.,  diazobenzene  nitrate, 

C(iH5 — N— N — HS04,  diazobenzene  sulphate, 

C6H. — N=N — OK,  diazobenzene  potassa,  [zene. 

C6H3— N=N— NH(C6H5)      diazobenzene  diamicloben- 

Azobenzene  is  formed  by  the  reduction  of  nitrobenzene. 
Its  formula  is  ClaH10N2.  As  nitrobenzene  contains  the 
group  C6H5  combined  with  N,  we  can  assume  that  azo- 
benzene  consists  of  two  such  groups  C6H5 — N=.  If 
these  combine  in  the  simplest  manner,  we  would  have 

C.H.-N 

the  formula  ||    ,    expressing  the  constitution  of 

C6H-N 

azobenzene.     This  is  the  formula  which  is  now  generally 
adopted. 

According  to  this,  the  azo-compounds  are  very  closely 
related  to  the  diazo-compounds.  Both  contain  the  group 
— N=N —  in  combination  with  C(1H5.  In'  reality,  the 
azo-compounds  differ  very  much  in  their  chemical  con- 
duct from  the  diazo-compounds.  The  decompositions 
which  they  undergo'  take  place  in  a  manner  entirely  dif- 
ferent from  that  already  -noticed  as  characterizing  the 
decomposition  of  diazo-compounds. 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        225 

This  difference  has  led  some  chemists  to  abandon  the 
formulas  above  given  for  the  diazo-compounds,  proposing 
in  their  place  others.     The  compounds  are  supposed  to 
be  ammonium  compounds  of  the  general  formula — 
R— N— R/ 

They  contain  one  quinquivalent  and  one 


I 


— 0—  N03  ;  N— 0— NO, 


trivalent  nitrogen  atom.      The  relation  between  anilin 
nitrate  and  diazobenzene  nitrate  is  shown  thus : — 

C6H6  CUH5 

J 

I' 

H3  N 

Anilin  nitrate.  Diazobenzene  nitrate. 

It  remains  to  be  determined,  by  future  experiments, 
which  of  the  formulas  for  diazo-compounds  is  correct. 
Up  to  the  present  there  exist  no  good  proofs  of  either. 

• 

NAPHTHALENE  DERIVATIVES. 

The  hydrocarbon  naphthalene  has  the  formula  C10H8. 
It  is  considered  as  being  formed  by  the  union  of  two 
benzene  nuclei,  and  as  having  the  constitution  expressed 
by  the  formula — 

H    H 

C— C 

S        \ 
HC  CH 

r\ r\ 

HC  CH 

C— C 
H    H 

This  formula  is  deduced  from  the  following  facts: 
There  is  a  derivative  of  naphthalene  known  as  dichlor- 
naphthoquinone,  which  has  the  formula  C10H4C1202. 
When  this  substance  is  oxidized,  it  yields  phthalic  acid, 


226  CHEMICAL    COMPOUNDS. 

which  is  a  bi-substitution  product  of  benzene.  We  see 
thus  that  those  carbon  atoms  in  dichlornaphthoquinone 
which  are  not  in  combination  with  chlorine  form  a  ben- 
zene nucleus,  so  that  we  might  write  the  formula  of  the 
compound  C6H4.C4C1202.  This  formula  does  not  tell  us 
in  what  manner  the  atoms  C4C1.202  are  united,  but,  by 
the  aid  of  another  experiment,  this  can  be  determined. 

When  dichlornaphthoquinone  (the  same  substance  used 
in  the  preceding  experiment)  is  treated  with  phosphorus 
pentachloride,  it  is  converted  into  pentachlornaphthalene, 
the  formula  of  which,  according  to  what  was  said  above, 
is  C6H3C1.C4C1+.  By  analogy,  we  would  expect  this  com- 
pound by  oxidation  to  yield  monochlorphthalic  acid  ;  it, 
however,  yields  tetrachlorphthalic  acid.  This  shows 
that  the  four  carbon  atoms  which  are  in  combination 
with  chlorine  form  part  of  a  benzene  nucleus,  as  well  as 
the  other  carbon  atoms  of  naphthalene.  It  is  thus  proved 
that  in  naphthalene  there  are  two  benzene  nuclei.  The 
only  formula  which  agrees  with  this  fact  is  the  one  above 
given. 

The  derivatives  of  naphthalene  resemble  those  of  ben- 
zene, and  much  that  has  been  said  concerning  this  latter 
would  hold  good  in  regard  to  the  former.  All  the  hydro- 
gen atoms  of  naphthalene  may  be  replaced  by  substi- 
tuting groups  or  elements,  and  thus,  as  will  be  readily 
seen,  a  large  number  of  substitution  products  may  be 
obtained.  The  possibilities  for  instances  of  isomerisrn 
are  greater  in  the  case  of  naphthalene  than  in  the  case  of 
benzene,  but  the  principles  governing  the  matter  of  iso- 
merism  are  essentially  the  same  as  those  which  we  have 
already  considered  in  connection  with  the  isomeric  sub- 
stitution products  of  benzene. 

Anthracene  Derivatives. — Anthracene,  like  naphtha- 
lene and  benzene,  is  the  mother-substance  of  a  large 
group  of  compounds.  Its  formula  is  014H10.  In  regard 
to  its  constitution,  the  view  is  now  commonly  held  that 
it  consists  of  two  benzene  nuclei,  C6H4,  held  together  by 

i    i 

means  of  the  group    HC — CH  ,    each    carbon   atom    of 

i    i 

which  is  united  with  both  benzene  nuclei,  thus: — 


CONSTITUTION    OF    CHEMICAL    COMPOUNDS.        227 

H    H 

C—  C 

/         \ 

HC  CH 


HC      CH 


HC  CH 

\        / 
C—  C 
H    H 

This  formula  shows  the  relation   between  anthracene 
and  anthraquinone,  which  latter  compound  appears  to  be 

CCK 
C6H/          /C6H4  .      The  latter  formula   best  explains 

\C(K 

the  formation  of  anthraquinone  from  benzoic  acid,  and 
the  formation  of  benzoic  acid  from  anthraquinone.  The 
former  transformation  is  represented  thus  :  — 

C.H.J1J  CO  |oH|  "  '       . 

=     C6H,<       >C.H.      +     2H,0. 
C.H4jH|  CO  OHj 

2  molecules  Benzoic  acid.  Anthraquinone. 

The  formation  of  anthraquinone  from  anthracene  would 
be  then  represented  thus:  — 

CH  C0 


According  to  these  interpretations,  anthraquinone  is  a 
double  acetone.  It  has  been  suggested,  further,  that  all 
quinones  are  nothing  but  double  acetones.  (See  Qui- 
nones,  p.  221.) 

The   derivatives  of  anthracene   resemble  in    some  re- 


228  CHEMICAL    COMPOUNDS. 

spects  those  of  naphthalene  and  of  benzene.  It  will  be 
seen,  however,  that  some  differences  must  exist  between 
them. 


Other  hydrocarbons  allied  to  naphthalene  are  pyrene, 
chrysene,  and  phenanthrene.  These  have  not  been  in- 
vestigated very  fully  as  compared  with  naphthalene  and 
anthracene  themselves.  All  of  these  three  undoubtedly 
contain  benzene  nuclei  as  essential  parts  of  their  mole- 
cules, but  there  is  still  some  doubt  in  regard  to  the 
manner  in  which  these  nuclei  are  united. 


INDEX. 


ACETONES,  142,  195 
Acetylene,  199 
Acids,  112 
Acids : 

acrylic,  198 

boric,  176 

butyric,  normal,  193 

caproic,  normal,  194 

caproic  ordinary,  194 

carbonic,  209 

of  chlorine,  163 

chromic,  181 

citric,  207 

cyanic,  208 

dithionic,  168 

gly eerie,  206 

glycolic,  202 

hypophosphorous,  172 

hyposulphurous,  167 

isobutyric,  194 

isocaproic,  195 

isophthalic,  216 

isosuccinic,  205 

lactic  (oxypropionic),  202 

metaphosphoric,  176 

nitric,  171 

nitrous,  171 

oxalic,  204 

pentathionic,  169 

phosphoric,  174 

phosphorous,  172 

phthalic,  216 

polysilicic,  177 

propionic,  192 

pseudocaproic,  195 

pyrochromic,  181 

pyrophosphoric,  175 

pyrosulphuric,  167 

sarcolactic,  203 

silicic.  177 
20 


Acids,  succinic,  205 

sulphuric,  166 

sulphurous,  165 

tartaric,  206 

terephthalic,  216 

tetrathionic,  169 

thiosulphuric,  167 

trithionic,  168 

uric,  211 

valeric,  normal,  194 

valeric,  ordinary,  194 
Acids  of  carbon,  135 
Acids,  hydrogen,  113 
Acids,  hydroxyl,  113 
Acids,  subdivision  of,  116 
Alcohols,  128 
Alcohols: 

allyl,  197 

amylene  hydrate,  188 

amyl  of  fermentation,  188 

amyl,  normal,  188 

amyl,  tertiary,  188 

butyl,  normal,  187 

butyl,  secondary,  187 

butyl,  tertiary,  187 

diethylmethylcarbinol,  190 

dimethylcarbinol,  190 

dimethylpseudopropylcarbi- 
nol,  190 

ethylene  (glycol),  201 

hexyl,  primary,  190 

hexyl,  secondary,  190 

isoamyl,  188 

isobutyl,  187 

propyl,  normal,  185 

pseudopropyl,  185 
Alcohols,  primary,  129 
Alcohols,  secondary,  130 
Alcohols,  tertiary,  132 
Aldehydes,  139,  195 


230 


INDEX. 


Alloys,  29 

Aluminium,  salts  of,  180 

Amido-group,  100 

Ammonium  salts,  100 

Ampere,  34 

Anhydrides,  119 

Anhydrides  of  acids  of  carbon,  147 

Anhydrides,  peculiar,  148 

Anthracene,  226 

Anthraquinone,  227 

Artiads,  92 

Atomic  compounds,  85 

Atomicity,  7  9^. 

Atomic  weights,  determination  of, 

18 

method  by  analysis,  19 
method  of  Berzelius,  22 
method  by  substitution,  24 
method  by  chemical   decom- 
position, 26 
by  Avogadro's  method,  32  ff. 

Atomic  theory,  17 

Atoms,  17,  36 

Avogadro,  33 


BASES,  117 
Benzene,  bisubstitution  pro- 
ducts, 215 

Benzene,  constitution  of,  212 
Benzene-nucleus,  215 
Berthelot,  15 
Berzelius,  22 
Boron,  an  exception  to  the  law  of 

Duloug  and  Petit,  64 
Boron,  the  element  unknown,  67 
Boron  trioxide,  176 
Butane,  derivatives  of,  185 


CARBON,  compounds  of,  124 
Carbon,  an  exception  to  the 
law  of  Dulongand  Petit,  64 
Carbylamines,  152 
Chemism,  defined,  14 
Chains,  108 
Chains,  closed,  109 
Chains,  open,  108 
Chromium,  salts  of,  17,9 
Clausius,  37 

Combination,  forms  of,  101 
Compounds,  defined,  28 


Constitution,  99 
Copper,  salts  of,  178 
Cyannmide,  208 
Cyanogen  compounds,  207 
Cyanogen  group,  152 


D ALTON,  investigations,  15 
Diazo-bodies,  222 
Double  union,  91 
Dulong,  58 
Dumas,  44 


T^LEMENTS,  defined,  27 
LJ     Equivalents,  20 
Ethane,  derivatives  of,  183 
Ethers,  145 
Ethers,  simple,  146 
Ethylene  chloride,  184 
Ethylene  derivatives,  196 
Ethylidene  chloride,  184 


FAVRE,  41 
Formulas,  molecular,  52 


GASES,  32 
Gay  Lussac,  32 
Glycerin,  205 
Glycols,  200 


HAMPE,  67 
Heptane,  derivatives  of,  190 
Hexane,  derivatives  of,  189 
Homologous  series,  126 
Humboldt,  32 
Hydrocarbons,  125 
Hydroxylamine,  171 


TMIDE  GROUP,  100 
1     Iron,  salts  of,  179 
Isomerism,  162 
Isomorphism,  68 
Isonitriles,  152 


J7-RONIG,  37 


INDEX. 


231 


T  AVOISIER,  14 
1J     Law 

of  Avogndro,  37 
of  Dulong  and  Petit,  58 
of  multiple  proportions,  16 
of  Neumann  and  Regnault, 

59 
periodic,  76 


MARRIOTTE,  36 
Maxwell,  37 

Mendelejeff,  scheme  of,  71 
Mercaptuns,  134 
Mercury,  salts  of,  178 
Mesitylene,  219 
Metal  acids,  180 
Meyer,  L.,  37,  44 
scheme  of,  77 
Mitscherlich,  44 
Mixtures,  mechanical,  29 
Molecular  compounds,  85 
Molecular  weights,  37 
Molecule,  36 
Mustard  oils,  208 


ATAPHTHALENE,  225 
11      Nascent  state,  50 
Naumann,  37 
Neumann,  58 
Nitriles,  152 
Nitrogen,  oxides  of,  170 
Nitrogen,  quinquivalent,  89 
Nitro  group.  156 
Nitroso  group,  159 


0 


XIDES,  123 
Ozone,  51 


pENTANE,  derivatives  of,  187 
JL      Periods   in    groups    of    ele- 
ments, 72 
Perissads,  92 
Petit,  58 
Pfaundler,  37 
Phenoles,  221 
Phosphorus,  oxides  of,  172 


Profane,  derivatives  of,  184 
Propylene,  197 
Proust,  15 


QUALITATIVE  METHOD,  14 
Quantitative  method,  14 
Quinones,  221 


REGNAULT,  58 
Residues,  105 


OALTS,  118,  178 

kJ     Saturated  compounds,  90 

Silbermann,  41 

Silicon,    an    exception    to   law  of 

Dulong  and  Petit,  64 
Solutions,  29 
Specific  heat,  56 
Substituting  groups,  constitution 

of,  151 

Substitution,  24 
Substitution  products,  148 
Sulpho  group,  154 


rpHOMSEN,  37 

1      Treble  union,  109 

Trimethylmethane,  186 

Types,  103 

Types,  theory  of,  104 


TTNSATURATEDCOMPOUNDS, 
U     90 

Uranium,  compounds  of,  181 
Urea,  210 


VALENCE,  79  /. 
Valence,  apparent,  97 
Valence,  true,  95 
Valence,  variable,  91 
Volume  of  gases,  32,  33 


w 


OLLASTON,  20 
Wxirtz,  93 


ERRATA. 

viv      i 
Page  84,  line  2  from  bottom,  omit  "  nitric  acid,  N02(OH)." 

Page  84,  bottom  line,  omit  "nitrates  and." 

Page  191,  middle  of  page,  read  "A  third  variety  of  heptane"  for 

"A  third  variety  of  hexane." 

Page  217,  middle  of  page,  after  the  sentence  ending  "  1  and  2  in 
this  formula,"  insert  "these  groups  being  considered 
in  their  relations  to  the  upper  benzene  nucleus." 

Page  227,  the  two  middle  carbon   atoms   in   the  formula  for 
anthracene  should  be  connected  by  a  line. 


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Prof,  of  Obstetrics,  etc.,  in  Coll.  of  Phys.  and  Surgeons,  N.  Y.  ;  J. 
S.  Billings,  M.D.,  U.S.A.,  Librarian  of  National  Medical  Library, 
Washington.  In  one  handsome  royal  12mo.  volume  of  366  pages. 
Cloth,  $2  25. 

PHRISTISON  (ROBERT).     DISPENSATORY  OR  COMMENTARY  ON 

V     THE    PHARMACOPOEIAS    OF    GREAT    BRITAIN    AND    THE 

UNITED  STATES.     With  a  Supplement  by  R.  E.  Griffith.     In  one 

8vo.  vol.  of  over  1000  pages,  containing  213  illustrations.     Cloth,  $4. 

pHURCHILL  (FLEETWOOB).    ON  THE  THEORY  AND  PRACTICE 
U     OF  MIDWIFERY.     With  notes  and  additions  by  D.  Francis  Condie, 
M.D.     With  about  200  illustrations.     In  one  handsome  8vo.  vol.  of 
nearly  700  pages.     Cloth,  $4  ;  leather,  $5. 

ESSAYS  ON  THE  PUERPERAL  FEVER,  AND  OTHER  DIS- 
EASES PECULIAR  TO  WOMEN.  In  one  neat  octavo  vol.  of  about 
450  pages.  Cloth,  $2  50. 

pDNDIE  (0.  FRANCIS).     A  PRACTICAL  TREATISE  ON  THE  DIS- 

^     EASES  OF  CHILDREN.     Sixth  edition,  revised  and  enlarged.     In 

one  large  8vo.  vol.  of  800  pages.     Cloth,  $5  25  ;  leather,  $6  25. 

pULLERIER  (1.)     AN  ATLAS  OF  VENEREAL  DISEASES.     Trans- 
^     lated  and  edited  by  FREEMAN  J.  BUMSTE  An,  M.D.     A  large  imperial 
quarto  volume,  with  26  plates  containing  about  150  figures,  beauti- 
fully colored,  many  of  them  the  size  of  life.     In  one  vol.,  strongly 
bound  in  cloth,  $17. 

Same  work,  in  five  parts,  paper  covers,  for  mailing,  $3  per  part. 


HENRY  C.  LEA'S  PUBLICATIONS. 


CYCLOPEDIA  OF  PRACTICAL  MEDICINE.  By  Dunglison,  Forbes, 
*J  Tweedie,  and  Conolly.  In  four  large  super-royal  octavo  volumes,  of 

3254  double-columned  pages,  leather,  raised  bands,  $15.  Cloth,  $11. 
p.\MPBELL'S  LIVES  OF  LORDS  KENYON.  ELLENBOROUGH,  AND 
W  TENTERDEN.  Being  the  third  volume  of  "  Campbell's  Lives  of 

the  Chief  Justices  of  England."    In  one  crown  octavo  vol.    Cloth,  $2, 

DA.LTON  (J.  C.)  A  TREATISE  ON  HUMAN  PHYSIOLOGY.  Sixth 
edition,  thoroughly  revised,  and  greatly  enlarged  and  improved,  with 
316  illustrations.  In  one  very  handsome  8vo.  vol.  of  830  pp. 
Cloth,  $5  50  ;  leather,  $6  50.  (Just  issued.) 

DAVIS  (F.  H.)     LECTURES  ON    CLINICAL   MEDICINE.     Second 
edition,  revised  and  enlarged.     In  one  12mo.  vol.       Cloth,  $1  75. 
DON  QUIXOTE  DE  LA  MANCHA.   Illustrated  edition.    In  two  hand- 
some vols.  crown  8vo.     Cloth,   $2  50  ;  half  morocco.  $3  70. 
DEWEES  (W.  P.)     A  TREATISE  ON  THE  DISEASES  OF  FEMALES. 
With  illustrations.     In  one  8vo.  vol.  of  536  pages.     Cloth,  $3. 

A  TREATISE  ON  THE  PHYSICAL  AND  MEDICAL  TREAT- 
MENT OF  CHILDREN.  In  one  8vo.  vol.  of  548  pages.  Cloth,  $2  80. 

DUTJITT  (ROBERT).  THE  PRINCIPLES  AND  PRACTICE  OF  MO 
DERN  SURGERY.  A  revised  American,  from  the -eighth  London 
edition.  Illustrated  with  432  wood  engravings.  In  one  8vo.  vol. 
of  nearly  700  pages.  Cloth,  $4 ;  leather,  $5. 

•HTINGLISON  (UOBLEY)  MEDICAL  LEXICON;  a  Dictionary  of 
-LJ  Medical  Science.  Containing  a  concise  explanation  of  the  various 
subjects  and  terms  of  Anatomy,  Physiology,  Pathology,  Hygiene, 
Therapeutics,  Pharmacology,  Pharmacy,  Surgery,  Obstetrics,  Medical 
Jurisprudence,  and  Dentistry.  Notices  of  Climate  and  of  Mineral 
Waters;  Formulae  for  Officinal,  Empirical,  and  Dietetic  Preparations, 
with  the  accentuation  and  Etymology  of  the  Terms,  and  the  French 
and  other  Synonymes.  In  one  very  large  royal  8vo.  vol.  New  edi- 
tion. Cloth,  $6  50;  leather,  $7  50.  (Just  issued.) 

HUMAN   PHYSIOLOGY.     Eighth  edition,  thoroughly  revised. 

In  two  large  8vo.  vols.  of  about  1500  pp.,  with  532  illus.     Cloth,  $7. 

NEW  REMEDIES,  WITH  FORMULAE  FOR  THEIR  PREPARA- 
TION AND  ADMINISTRATION.  Seventh  edition.  In  one  very 
large  8vo.  vol.  of  770  pages.  Cloth,  $4. 

DE  LA  BECHE'S  GEOLOGICAL  OBSERVER.  In  one  large  8vo.  vol. 
of  700  pages,  with  300  illustrations.  Cloth,  $4. 

DANA  (JAMES  D.)  THE  STRUCTURE  AND  CLASSIFICATION  OF 
ZOOPHYTES.  With  illustrations  on  wood.  In  one  imperial  4to.  vol. 
Cloth,  $4  00. 

•PLLIS    (BENJAMIN).      THE    MEDICAL    FORMULARY.      Being   a 
•"     collection  of  prescriptions  derived  from  the  writings  and  practice  of 
the  most  eminent  physicians  of  America  and  Europe.     Twelfth  edi- 
tion, carefully  revised  by  A.  H.  Smith,  M.  D.     In  one  8vo.  volume 
of  374  pages.     Cloth,  $3. 

ERICHSEN  (JOHN).  THE  SCIENCE  AND  ART  OF  SURGERY. 
A  new  and  improved  American,  from  the  sixth  enlarged  and  re- 
vised London  edition.  Illustrated  with  630  engravings  on  wood.  In 
two  large  8vo.  vols.  Cloth,  $9  00;  leather,  raised  bands,  $11  00. 

ENCYCLOPAEDIA  OF  GEOGRAPHY.     In  three  large  8vo.  vols.     Illus- 
trated with  83  maps -and  about  1100  wood-cuts.     Cloth,  $5. 
pOTHERGILL'S  PRACTITIONER'S  HANDBOOK  OF  TREATMENT. 
•*•      In  one  handsome  octavo  volume.     (  in  press.) 


HENRY  C.  LEA'S  PUBLICATIONS.  5 

pENWICK    (SAMUEL).     THE   STUDENTS'   GUIDE   TO  MEDICAL 
J-      DIAGNOSIS.     From  the  Third  Revised  and  Enlarged  London  Edi- 
tion.    In  one  vol.  royal  12rao.     Cloth,  $2  25. 

FLETCHER'S  NOTES  FROM  NINEVEH,  AND  TRAVELS  IN  MESO. 
POTAMIA,  ASSYRIA,  AND  SYRIA.  In  one  12mo.  vol.  Cloth,  75  cts. 
FOX  ON  DISEASES  OF  THE  STOMACH.     From  the  third  London  edi- 
tion.    In  one  octavo  vol.     Cloth,  $2.     (Jiist  issued.) 
pOX  (TILBURY).     EPITOME  OF  SKIN  DISEASES,  with   Formula? 
•*•      for  Students  and  Practitioners.     In  one  small  12mo.  vol.     Cloth,  $1. 
PLINT    (AUSTIN).     A    TREATISE    ON    THE    PRINCIPLES    AND 
•L      PRACTICE  OF  MEDICINE.     Fourth  edition,  thoroughly  revised 
and  enlarged.     In  one  large  8vo.  volume  of  1070  pages.     Cloth,  $6  ; 
leather,  raised  hands,  $7.     (Just  issued.) 

A  PRACTICAL  TREATISE  ON  THE  PHYSICAL  EXPLORA- 
TION OF  THE  CHEST,  AND  THE  DIAGNOSIS  OF  DISEASES 
AFFECTING  THE  RESPIRATORY  ORGANS.    Second  and  revised 
edition.     One  8vo.  vol.  of  595  pages.     Cloth,  $4  50. 

A  PRACTICAL  TREATISE  ON  THE  DIAGNOSIS  AND  TREAT- 
MENT OF  DISEASES  OF  THE  HEART.  Second  edition,  enlarged. 
In  one  neat  8vo.  vol.  of  over  500  pages,  $4  00. 

ON  PHTHISIS  :  ITS  MORBID  ANATOMY,  ETIOLOGY,  ETC., 

in  a  series  of  Clinical  Lectures.    A  new  work.    In  one  handsome  8vo. 
volume.     Cloth,  $3  50.      (Just  issued.) 

A  MANUAL  OF  PERCUSSION  AND  AUSCULTATION ;  of  the 

Physical  Diagnosis  of  Diseases  of  the  Lungs  and  Heart,  and  of  Tho- 
racic Aneurism.  In  one  handsome  royal  12mo.  volume.  Cloth, 
$1  75.  (Now  ready.) 

MEDICAL  ESSAYS.     In  one  neat  12mo.  volume.     Cloth,  $1  38. 


pOWNES  (GEORGE).  A  MANUAL  OF  ELEMENTARY  CHEMISTRY. 
•*-      From  the  tenth  enlarged  English  edition.     In  one  royal  12mo.  vol.  of 

857  pages,  with  197  illustrations.     Cloth,  $2  75  ;  leather,  $3  25. 
PULLER  (HENRY).    ON   DISEASES  OF  THE   LUNGS  AND  AIR 
-L      PASSAGES.     Their  Pathology,  Physical  Diagnosis,  Symptoms,  and 

Treatment.     From  the  second  English  edition.     In  one  8vo.  vol. 

of  about  500  pages.     Cloth,  $3  50. 

GALLOWAY  (ROBERT).  A  MANUAL  OF  QUALITATIVE  AN- 
ALYSIS. From  the  fifth  English  edition.  In  one  12mo.  vol.  Cloth, 
$2  50.  (Lately published.) 

GLUGE  (GOTTLIEB).  ATLAS  OF  PATHOLOGICAL  HISTOLOGY. 
Translated  by  Joseph  Leidy,  M.D.,  Professor  of  Anatomy  in  the 
University  of  Pennsylvania,  &c.  In  one  vol.  imperial  quarto,  with 
320  copperplate  figures,  plain  and  colored.  Cloth,  $4. 

GREEN  (I.  HENRY).  AN  INTRODUCTION  TO  PATHOLOGY  AND 
MORBID  ANATOMY.  Second  Arner.,  from  the  third  Lond.  Ed. 
In  one  handsome  8vo.  vol.,  with  numerous  illustrations.  Cloth, 
$2  75.  (Just  issued.) 

GRAY  (HENRY).  ANATOMY,  DESCRIPTIVE  AND  SURGICAL. 
A  new  American,  from  the  fifth  and  enlarged  London  edition.  In  one 
large  imperial  8vo.  vol.  of  about  900  pages,  with  462  large  and  elabo- 
rate engravings  on  wood.  Cloth,  $6;  leather,  $7.  (Lately  issued.) 

GRIFFITH  (ROBERT  E.)  A  UNIVERSAL  FORMULARY,  CON- 
TAINING THE  METHODS  OF  PREPARING  AND  ADMINISTER- 
ING OFFICINAL  AND  OTHER  MEDICINES.  Third  and  Enlarged 
edition.  Edited  by  John  M.  Maisch.  In  one  large  8vo.  vol.  of  800 
pages,  double  columns.  Cloth,  $4  50  ;  leather,  $5  50. 
# 


HENRY  C.  LEA'S  PUBLICATIONS. 


GROSS  (SAMUEL  D.)  A  SYSTEM  OF  SURGERY,  PATHOLOGICAL, 
DIAGNOSTIC,  THERAPEUTIC,  AND  OPERATIVE.  Illustrated 
by  1403  engravings.  Fifth  edition,  revised  and  improved.  In  two 
large  imperial  8vo.  vols.  of  over  2200  pages,  strongly  bound  in 
leather,  raised  bands,  $15. 

GROSS  (SAMUEL  D.)  A  PRACTICAL  TREATISE  ON  THE  DIS- 
eases,  Injuries,  and  Malformations  of  the  Urinary  Bladder,  the  Pros- 
tate Gland,  and  the  Urethra.  Third  Edition,  'thoroughly  Revised 
and  Condensed,  by  Samuel  W.  Gross,  M.D.,  Surgeon  to  the  Phila- 
delphia Hospital.  In  one  handsome  octavo  volume,  with  about  two 
hundred  illustrations.  Cloth,  $4  50.  (Now  ready.) 

A  PRACTICAL  TREATISE  ON  FOREIGN  BODIES  IN  THE 

AIR  PASSAGES.    In  one  8vo.  vol.  of  468  pages.    Cloth,  $2  75. 

pIBSON'S  INSTITUTES  AND  PRACTICE  OF  SURGERY.    In  two  8vo. 
vJ    vols.  of  about  1000  pages,  leather,  $6  50. 

GOSFELIN(L)  CLINICAL  LECTURES  ON  SURGERY,  Delivered 
at  the  Hospital  of  La  Charite  Translated  from  the  French  by  Lewis 
A.  Stirmon,  M.D.,  Surgeon  to  the  Presbyterian  Hospital,  New  York. 
With  illustrations.  (Publishing  in  the  Medical  News  and  Library 
for  1876-7.) 

HUDSON  (A..)  LECTURES  ON  THE  STUDY  OF  FEVER.  1  vol. 
8vo.,  316  pages.  Cloth,  $2  50. 

HEATH  (CHRISTOPHER).  PRACTICAL  ANATOMY  ;  A  MANUAL 
OF  DISSECTIONS.  With  additions,  by  W.  W.  Keen,  M.  D.  In  1 
volume  ;  with  247  illustrations.  Cloth,  $3  50  ;  leather,  $4. 

HARTSHORNE  (HENRY).  ESSENTIALS  OF  THE  PRINCIPLES 
AND  PRACTICE  OF  MEDICINE.  Fourth  and  revised  edition. 
In  one  12ino.  vol.  Cloth,  $2  63;  half  bound,  $2  88.  (Lately  issued  ) 

CONSPECTUS  OF  THE  MEDICAL  SCIENCES.  Comprising 

Manuals  of  Anatomy,  Physiology,  Chemistry,  Materia  Medica,  Prac- 
tice of  Medicine,  Surgery,  and  Obstetrics.  Second  Edition.  In  one 
royal  12mo.  volume  of  over  1000  pages,  with  477  illustrations. 
Strongly  bound  in  leather,  $5  00  ;  cloth,  $4  25.  (Lately  issued.) 

A  HANDBOOK  OF  ANATOMY  AND  PHYSIOLOGY.  In  one 

neat  royal  12mo.  volume,  with  many  illustrations.  Cloth,  $1  75. 

HAMILTON  (FRANK  H.)  A  PRACTICAL  TREATISE  ON  FRAC- 
TURES AND  DISLOCATIONS.  Fifth  edition,  carefully  revised. 
In  one  handsome  8vo.  vol.  of  830  pages,  with  344  illustrations.  Cloth, 
$5  75  ;  leather,  $5  75.  (Just  issued.) 

HOLMES  (TIMOTHY).  SURGERY,  ITS  PRINCIPLES  AND  PRAC- 
TICE. In  one  handsome  8vo.  volume  of  1000  pages,  with  411  illus- 
trations. Cloth,  $6;  leather,  with  raised  bands,  $7.  ( Jit st  ready.) 
HOBLYN  (RICHARD  D.)  A  DICTIONARY  OF  THE  TERMS  USED 
IN  MEDICINE  AND  THE  COLLATERAL  SCIENCES.  In  one 
12mo.  volume,  of  over  500  double-columned  pages.  Cloth,  $1  50 
leather,  $2. 

HODGE  (HUGH  L.)  ON  DISEASES  PECULIAR  TO  WOMEN,  IN 
CLUDING  DISPLACEMENTS  OF  THE  UTERUS.  Second  and 
revised  edition.  In  one  8vo.  volume.  Cloth,  $4  50. 

THE  PRINCIPLES  AND  PRACTICE  OF  OBSTETRICS.    Illus 

trated  with  large  lithographic  plates  containing  159  figures  from 
original  photographs,  and  with  numerous  wood-cuts.  In  one  large 
quarto  vol.  of  550  double-columned  pages.  Strongly  bound  in  cloth 
$14. 


HENRY  C.  LEA'S  PUBLICATIONS. 


H 


HOLLAND  (SIR  HENRY).  MEDICAL  NOTES  AND  REFLECTIONS. 
From  the  third  English  edition.  In  one  8vo.  vol.  of  about  500  pages. 
Cloth,  $3  50. 

HODGES  (RICHARD  M.)  PRACTICAL  DISSECTIONS.  Second  edi- 
tion. In  one  neat  royal  12ino.  vol.,  half  bound,  $2. 

HUGHES.  SCRIPTURE  GEOGRAPHY  AND  HISTORY,  with  12 
colored  maps.  In  1  vol.  12mo.  Cloth,  $1. 

TT3RNER  (WILLIAM  E.)     SPECIAL  ANATOMY  AND  HISTOLOGY. 
-*-L   Eighth  edition,  revised  and  modified.     In  two  large  8vo.  vols.  of  over 
1000  pages,  containing  300  wood-cuts.     Cloth,  $6. 

HILL  (BERKELEY).  SYPHILIS  AND  LOCAL  CONTAGIOUS  DIS- 
ORDERS. In  one  8vo.  volume  of  467  pages.  Cloth,  $3  25. 

SILLIER  (THOMAS).  HAND-BOOK  OF  SKIN  DISEASES.  Second 
Edition.  In  one  neat  royal  12mo.  volume  of  about  300  pp. ,  with  two 
plates.  Cloth,  $2  25 

ALL  (MRS.  M.)  LIVES  OF  THE  QUEENS  OF  ENGLAND  BEFORE 
THE  NORMAN  CONQUEST.  In  one  handsome  8vo.  vol.  Cloth, 
$2  25;  crimson  cloth,  $2  50;  half  morocco,  $3. 

JONES  (C.  HANDFIELD).  CLINICAL  OBSERVATIONS  ON  FUNC- 
TIONAL NERVOUS  DISORDERS.  Second  American  Edition.  In 
one  8vo.  vol.  of  348  pages.  Cloth,  $3  25. 

EIRKES  (WILLIAM  SENHOUSE).  A  MANUAL  OF  PHYSIOLOGY. 
A  new  American,  from  the  eighth  London  edition.  One  vol.,  with 
many  illus.,  12mo.  Cloth,  $3  25;  leather,  $3  75. 

KNAPP  (F.)  TECHNOLOGY  ;  OR  CHEMISTRY,  APPLIED  TO  THE 
ARTS  AND  TO  MANUFACTURES,  with  American  additions,  by 
Prof.  Walter  R.  Johnson.  In  two  8vo.  vols.,  with  500  ill.  Cloth,  $6. 

KENNEDY'S  MEMOIRS  OF  THE  LIFE  OF  WILLIAM  WIRT.  In 
two  vols.  12mo.  Cloth,  $2. 

T  EA  (HENRY  C.)    SUPERSTITION  AND  FORCE  ;  ESSAYS  ON  THE 
-Ll     WAGER  OF  LAW,  THE  WAGER  OF  BATTLE,  THE  ORDEAL, 

AND  TORTURE.    Second  edition,  revised.     In  one  handsome  royal 

12mo.  vol.,  $2  75. 
STUDIES  IN  CHURCH  HISTORY.     The  Rise  of  the  Temporal 

Power — Benefit  of  Clergy — Excommunication.      In   one   handsome 

12mo.  vol.  of  515  pp.     Cloth,  $2  75. 
AN  HISTORICAL   SKETCH   OF   SACERDOTAL  CELIBACY 

IN  THE  CHRISTIAN  CHURCH.     In  one  handsome  octavo  volume 

of  602  pages.     Cloth,  $3  75. 

LA  ROCHE  (R.)  YELLOW  FEVER.  In  two  8vo.  vols.  of  nearly  1500 
pages.  Cloth,  $7. 

PNEUMONIA.    In  one  8vo.  vol.  of  500  pages.     Cloth,  $3. 

T  INCOLN  (D.  F.)     ELECTRO-THERAPEUTICS.     A  Condensed  Man- 
£-1    ual  of  Medical  Electricity.     In  one  neat  royal  12mo.  volume,  with 

illustrations.     Cloth,  $1  50.     (Just  issued.) 

TEISHMAN  (WILLIAM).     A  SYSTEM  OF  MIDWIFERY.     Includ- 
Ll     ing  the  Diseases  of  Pregnancy  and  the  Puerperal  State.      Second 
American,  from  the  Second  English  Edition.     With  ndditions,  by 
J.  S.  Parry,  M.D.     In  one  very  handsome  8vo.  vol.  of  800  pages  and 
200  illustrations.     Cloth,  $5  ;   leather,  $6.     (Just  issued.) 
r  AURENCE  (J.  Z.)   AND  MOON  (ROBERT  C.)     A   HANDY-BOOK 
LI     OF  OPHTHALMIC  SURGERY.     Second  edition,  revised  by  Mr. 
Laurence.     With  numerous  illus.     In  one  8vo.  vol.     Cloth,  $2  75. 


HENRY  C.  LEA'S  PUBLICATIONS. 


T  EHMANN  (C.  G.)     PHYSIOLOGICAL  CHEMISTRY.    Translated  by 
•LI     George  F.  Day,   M.  D.     With  plates,  and  nearly  200  illustrations. 

In  two  large  8vo.  vols.,  containing  1200  pages.     Cloth,  $6. 
A    MANUAL   OF    CHEMICAL   PHYSIOLOGY.     In    one   very 

handsome  8vo.  vol.  of  336  pages.     Cloth,  $2  25. 

T  AWSON  (GEORGE).    INJURIES  OF  THE  EYE,  ORBIT,  AND  EYE- 
•Ll     LIDS,  with  about  100  illustrations.     From  the  last  English  edition. 
In  one  handsome  8vo.  vol.     Cloth,  $3  50. 

T  UDLOW  (J.  I.)     A  MANUAL  OF  EXAMINATIONS  UPON  ANA- 
-Ll     TOMY,  PHYSIOLOGY,  SURGERY,  PRACTICE  OF  MEDICINE, 

OBSTETRICS,  MATERIA  MEDICA,  CHEMISTRY,  PHARMACY, 

AND  THERAPEUTICS.     To  which  is  added  a  Medical  Formulary. 

Third  edition.     In  one  royal  12mo.  vol.  of  over  800  pages.     Cloth 

$3  25  ;   leather,  $3  75. 

TAYCOCK    (THOMAS).     LECTURES   ON   THE   PRINCIPLES    AND 
•LI     METHODS  OF  MEDICAL  OBSERVATION  AND  RESEARCH.    In 

one  12mo.  vol.     Cloth,  $1. 

T  YNCH  (W.  F.)     A  NARRATIVE  OF  THE  UNITED  STATES  EX- 
J-l     PEDITION  TO  THE  DEAD  SEA  AND  RIVER  JORDAN.     In  one 

large  octavo  vol.,  with  28  beautiful  plates  and  two  maps.    Cloth,  $3. 

Same  Work,  condensed  edition.    One  vol.  royal  12mo.    Cloth,  $1. 

T  EE   (HENRY)  ON  SYPHILIS.     In  one  8vo.  vol.     Cloth,  $2  25. 

T  YONS  (ROBERT  D.)     A  TREATISE  ON  FEVER.     In  one  neat  8vo. 
-LI     vol.  of  362  pages.     Cloth,  $2  25. 

MARSHALL    (JOHN).      OUTLINES    OF    PHYSIOLOGY,    HUMAN 
AND  COMPARATIVE.     With  Additions  by  FRANCIS   G.  SMITH, 
M.  D.,  Professor  of  the  Institutes  of  Medicine  in  the  University  of 
Pennsylvania.     In  one  8vo.  volume  of  1026  pages,  with  122  illustra- 
tions.    Strongly  bound  in  leather,  raised  bands,  $7  50.    Cloth,  $6  50. 
MACLISE  (JOSEPH).     SURGICAL   ANATOMY.     In  one  large   im- 
perial quarto  vol.,  with  68  splendid  plates,  beautifully  colored;  con- 
taining 190  figures,  many  of  them  life  size.     Cloth,  $14. 
MfiIGS  (CHAS.  D.).    ON  THE  NATURE,  SIGNS,  AND  TREATMENT 
OF  CHILDBED  FEVER.     In  one  8vo.  vol.  of  365  pages.     Cloth,  $2. 
TV/TILLER  (JAMES).    PRINCIPLES  OF  SURGERY.    Fourth  American, 
1YL  from  the  third  Edinburgh  edition.      In  one  large  8vo.  vol.  of  700 
pages,  with  240  illustrations.     Cloth,  $3  75. 

THE  PRACTICE  OF  SURGERY.     Fourth  American,  from  the 

last  Edinburgh  edition.  In  one  large  8vo.  vol.  of  700  pages,  with 
364  illustrations.  Cloth,  $3  75. 

MONTGOMERY  (W.  F.)  AN  EXPOSITION  OF  THE  SIGNS  AND 
SYMPTOMS  OF  PREGNANCY.  From  the  second  English  edition. 
In  one  handsome  8vo.  vol.  of  nearly  600  pages.  Cloth,  $3  75. 

MULLER  (J.)     PRINCIPLES  OF  PHYSICS  AND  METEOROLOGY. 
In  one   large  8vo.  vol.  with  550  wood-cuts,  and  two  colored  plates. 
Cloth,  $4  50. 
TUTIRA.BEAU  ;   A  LIFE  HISTORY.     In  one  12mo.  vol.     Cloth,  75  cts. 

MACFARLAND'S  TURKEY  AND  ITS  DESTINY.  In  2  rols.  royal 
12mo.  Cloth,  $2. 

MARSH  (MRS.)     A  HISTORY  OF  THE  PROTESTANT  REFORMA- 
TION IN  FRANCE.     In  2  vols.  royal  12mo.     Cloth,  $2. 
•M-ELIGAN  (J.  MOORE) .  AN  ATLAS  OF  CUTANEOUS  DISEASES.  In 
J-'     one  quarto  volume,  with  beautifully  colored  plates,  &c.  Cloth,  $5  50. 


HENRY  C.  LEA'S  PUBLICATIONS.  9 

TVTEILL   (JOHN)   AND  SMITH    (FRANCIS  G.)     COMPENDIUM  OF 
IN    THE  VARIOUS  BRANCHES  OF  MEDICAL  SCIENCE.     In  one 

handsome    12mo.   vol.   of  about   1000   pages,   with  374   wood-cuts. 

Cloth,  $4;  leather,  raised  bands,  $4  75. 

NIEBTJHR  (B.  G.)  LECTURES  ON  ANCIENT  HISTORY;  com- 
prising the  history  of  the  Asiatic  Nations,  the  Egyptians, 
Greeks,  Macedonians,  and  Carthagenians.  Translated  by  Dr.  L. 
Schmitz.  In  three  neat  volumes,  crown  octavo.  Cloth,  $500. 

ODLING  (WILLIAM).  A  COURSE  OF  PRACTICAL  CHEMISTRY 
FOR  THE  USE  OF  MEDICAL  STUDENTS.  In  one  12mo.  vol. 
of  261  pp.,  with  75  illustrations.  Cloth,  $2. 

PLAYFAIS  (W.  S  )  A  TREATISE  ON  THE  SCIENCE  AND  PRAC- 
TICE OF  MIDWIFERY.  In  one  handsome  octavo  vol.  of  576  pp., 
with  166  illustrations,  and  two  plates.  Cloth,  $4;  leather,  $5. 
(Jttst  issued.) 

PAVY  (F.  W.)  A  TREATISE  ON  THE  FUNCTION  OF  DIGESTION, 
ITS  DISORDERS  AND  THEIR  TREATMENT.  From  the  second 
London  ed.  In  one  8vo.  vol.  of  246  pp.  ^Cloth,  $2. 

A  TREATISE    ON    FOOD  AND   DIETETICS,  PHYSIOLOGI- 
CALLY AND  THERAPEUTICALLY  CONSIDERED.     In  one  neat 
octavo  volume  of  about  500  pages.     Cloth,  $4  75.     (Just  issued.) 
p.\RRISH  (EDWARD).     A  TREATISE  ON  PHARMACY.    With  many 
•L      Formulae  and  Prescriptions.  Fourth  edition.  Enlarged  and  thoroughly 
revised  by  Thomas  S.  Wiegand.     In  one  handsome  8vo.  vol.  of  977 
pages,  with  280  illus.     Cloth,  $5  50  ;  leather,  $6  50. 
pIRRIE  (WILLIAM)      THE  PRINCIPLES  AND  PRACTICE  OF  SUR- 
-L      GERY.     In  one  handsome  octavo  volume  of  780  pages,  with  316 

illustrations.     Cloth,  $3  75. 

pEREIRA  (JONATHAN).     MATERIA  MEDIC  A  AND  THERAPEU- 
•L      TICS.     An  abridged  edition.     With  numerous  additions  and  refe- 
rences to  the  United  States  Pharmacopoeia.      By  Horatio  C.   Wood, 
M.  D.     In  one  large  octavo  volume,  of  1040  pages,  with  236  illustra- 
tions.    Cloth,  $7  00;  leather,  raised  bands,  $8  00. 
PTJLSZKY'S  MEMOIRS  OF  AN  HUNGARIAN  LADY.     In   one  neat 
royal  12mo.  vol.     Cloth,  $1. 

PAGET'S  HUNGARY  AND  TRANSYLVANIA.     In  two  royal  12mo. 
-L      vols.     Cloth,  $2. 

TDEMSEN  (IRA).     THE  PRINCIPLES  OF  CHEMISTRY.     An  Intro- 
•t«    ductior-  to  Modern  Chemistry,  for  the  Use  of  Students.    In  one  12mo. 
vol.,  cloth.      (In press.) 

ROBERTS  (WILLIAM).     A  PRACTICAL  TREATISE  ON  URINARY 
AND  RENAL  DISEASES.     A  second  American,  from  the  second 
London  edition.     With  numerous  illustrations  and  a  colored  plate. 
In  one  very  handsome  8vo.  vol.  of  616  pages.     Cloth,  $4  51. 
•RA.MSBOTHAM   (FRANCIS   H.)     THE   PRINCIPLES  AND    PRAC- 
Iw    TICE  OF  OBSTETRIC  MEDICINE  AND  SURGERY.     In  one  im- 
perial 8vo.  vol.  of  650  pages,  with  64  plates,  besides  numerous  wood- 
cuts in  the  text.     Strongly  bound  in  leather,  $7. 

RIGBY  (EDWARD).     A  SYSTEM  OF  MIDWIFERY.     Second  Ameri. 
can  edition.    In  one  handsome  8vo.  vol.  of  422  pages.     Cloth,  $2  50. 
RANKE'S  HISTORY  OF  THE  TURKISH  AND  SPANISH  EMPIRES 
in  the  16th  and  beginning  of  17th  Century.     In  one  8vo.  volume, 
paper,  25  cts. 

HISTORY  OF  THE  REFORMATION  IN  GERMANY.     Parts  I., 

II.,  III.     In  one  vol.     Cloth,  $1. 


10  HENRY  C.  LEA'S  PUBLICATIONS. 

qCHAFER  (EDWAKD  ALBERT)      A  COURSE  OF  PRACTICAL  HIS- 

O      TOLOGF  :   A  Manual  of  the  Microscope  for  Medical  Students.     In 

one  handsome  octavo  vol.    With  numerous  illustrations.    (In press.) 

SMITH  (EUSTACE).  ON  THE  WASTING  DISEASES  OF  CHILDREN 
Second  American  edition,  enlarged.     In  one  8vo.  vol.     Cloth,  $2  50. 

OARGENT  (F.  W.)     ON  BANDAGING  AND  OTHER  OPERATIONS 
O     OF  MINOR  SURGERY.     New  edition,  with  an  additional  chapter 

on  Military  Surgery.     In  one  handsome  royal  12mo.  vol.  of  nearly 

400  pages,  with  184  wood-cuts.     Cloth,  $1  75. 

QMITH  (J.  LEWIS.)     A  TREATISE  ON  THE   DISEASES   OF   IN- 
W     FANCY  AND  CHILDHOOD.     Third  Edition,  revised  and  enlarged. 

In  one  large  8vo.  volume  of  724  pages,  with  illustrations.     Cloth, 

$5  ;  leather,  $6.     (Just  issued.) 

QHARPEY    (WILLIAM)    AND    QTJAIN    (JONES   AND    RICHARD) . 
O     HUMAN  ANATOMY.     With  notes  and  additions  by  Jos.    Leidy, 

M.D.,  Prof,  of  Anatomy  in  the  University  of  Pennsylvania.     In  two 

large  Svo.vols.  of  about  J  300  pages,  with  51 1  illustrations.     Cloth,  $6. 

SKEY  (FREDERIC  C.)  OPERATIVE  SURGERY.  In  one  8vo.  vol. 
of  over  650  pages,  with  about  100  wood-cuts.  Cloth,  $3  25. 

SLADE  (D.  D.)  DIPHTHERIA  ;  ITS  NATURE  AND  TREATMENT. 
Second  edition.  In  one  neat  royal  12mo.  vol.  Cloth,  $1  25. 

SMITH  (HENRY  H.)  AND  HORNER  (WILLIAM  E.)  ANATOMICAL 
ATLAS.  Illustrative  of  the  structure  of  the  Human  Body.  In  one  large 
imperial  8vo.  vol.,  with  about  650  beautiful  figures.  Cloth,  $4  50. 

SMITH  (EDWARD).  CONSUMPTION;  ITS  EARLY  AND  REME- 
DIABLE STAGES.  In  one  8vo.  vol.  of  254  pp.  Cloth,  $2  25. 

STILLE  (ALFRED).  THERAPEUTICS  AND  MATERIA  MEDIC  A. 
Fourth  edition,  revised  and  enlarged.  In  two  large  and  handsome 
volumes  8vo.  Cloth,  $10  ;  leather,  $12.  (Jitst  issued.) 

SCHMITZ  AND  ZUMPT'S  CLASSICAL  SERIES.     In  royal  18mo. 
CORNELII  NEPOTIS  LIBER  DE  EXCELLENTIBUS  DUCIBUS 
EXTERARUM  GENTIUM,  CUM  VITIS  CATONIS  ET  ATTICI. 
With  notes,  Ac.     Price  in  cloth,  60  cents;  half  bound,  70  cts. 

C.  I.  CJESARIS  COMMENTARII  DE  BELLO  GALLICO.  With  notes, 

map,  and  other  illustrations.     Cloth,  60  cents;  half  bound,  70  cents. 
C.  C.  SALLUSTII  DE  BELLO  CATILINARIO  ET  JUGURTHINO. 

With  notes,  map,  Ac.     Price  in  cloth,  60  cents  ;  half  bound,  70  cents. 
Q.  CURTII  RUFII  DE  GESTIS  ALEXANDRI  MAGNI  LIBRI  VIII. 

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P.  VIRGILII   MARONIS   CARMINA   OMNIA.     Price  in  cloth,  85 

cents;  half  bound,  $1. 
M.  T.  CICERONIS  ORATIONES  SELECTJE  XII.     With  notes,  Ac. 

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ADVANCED    LATIN    EXERCISES,     WITH     SELECTIONS    FOR 

READING.     Revised.     Cloth,  price  60  cents  ;  half  bound,  70  cents. 

S WAYNE  (JOSEPH  GRIFFITHS).  OBSTETRIC  APHORISMS.  A 
new  American,  from  the  fifth  revised  English  edition.  With  addi- 
.tions  by  E.  R.  Hutchins,  M.  D.  In  one  small  12mo.  vol.  of  177  pp., 
with  illustrations.  Cloth.  $1  25. 


HENRY  C.  LEA'S  PUBLICATIONS.  11 


QTURGES    (OCTAVITTF).     AN    INTRODUCTION    TO   THE    STUDY 
W     OF  CLINICAL   MEDICINE.     In  one  12mo.  vol.      Cloth,  $1   25. 

SCHOEDLER  (FREDERICK)  AND  MEDLOCK  (HENRY).  WONDERS 
OF  NATURE.  An  elementary  introduction  to  the  Sciences  of  Physics, 
Astronomy,  Chemistry,  Mineralogy,  Geology,  Botany,  Zoology, 
and  Physiology.  Translated  from  the  German  by  II.  Medlock.  In 
one  neat  8vo.  vol.,  with  679  illustrations.  Cloth,  $3. 

gTOKES  (W.)     LECTURES  ON  FEVER.    In  one  8vo.  vol.    Cloth,  $2. 

OMALL  BOOKS  ON  GREAT  SUBJECTS.     Twelve  works;  each  one  10 
^     cents,  sewed,  forming  a  neat  and  cheap  series  ;  or  done  up  in  3  vols., 

cloth,  $1  50. 

STRICKLAND    (AGNES).     LIVES  OF  THE  QUEENS  OF   HENRY 
KJ     THE  VIII.  AND  OF  HIS   MOTHER.     In  one  crown  octavo  vol., 

extra  cloth,  $1 ;  black  cloth,  90  cents. 
MEMOIRS  OF  ELIZABETH,  SECOND  QUEEN  REGNANT  OF 

ENGLAND  AND  IRELAND.     In  one  crown  octavo  vol.,  extra  cloth, 

$140;  black  cloth,  $1  30. 

TANNER  (THOMAS  HAWKES).  A  MANUAL  OF  CLINICAL  MEDI- 
CINE AND  PHYSICAL  DIAGNOSIS.  Third  American  from  the 
second  revised  English  edition.  Edited  by  Tilbury  Fox,  M.  D.  In 
one  handsome  12mo.  volume  of  366  pp.  Cloth,  $1  50. 

ON  THE  SIGNS  AND  DISEASES  OF  PREGNANCY.     From 

the  second  English  edition.     With  four  colored  plates  and  numerous 
illustrations  on  wood.     In  one  vol.  8vo.  of  about  500  pages.     Cloth, 
$4  25. 

rpUKE  (DANIEL  HACK).    INFLUENCE  OF  THE  MIND  UPON  THE 
•*•     BODY.     In  one  handsome  8vo.  vol.  of  416  pp.    Cloth,  $3  25.     (Just 

issued.) 

rPAYLOR    (ALFRED    S.)     MEDICAL    JURISPRUDENCE.     Seventh 
J-     American  edition.     Edited  by  John  J.  Reese,  M.D.     In  one  large 
8vo.  volume  of  879  pages.     Cloth,  $5;  leather,  $6.      (Just  issued.) 
PRINCIPLES  AND  PRACTICE   OF   MEDICAL   JURISPRU- 
DENCE.    From  the  Second  English  Edition.      In  two  large  8vo. 
vols.     Cloth,  $10;  leather,  $12.      (Juit  issued.) 

ON  POISONS  IN  RELATION  TO  MEDICINE  AND  MEDICAL 

JURISPRUDENCE.     Third  American  from  the  Third  London  Edi- 
tion.    1  vol.  8vo.  of  788  pages,  with  104  illustrations.     Cloth,  $5  50  ; 
leather,  $6  50.      (Just  issued.) 

rpHOMAS  (T.  GAILLARD).    A   PRACTICAL  TREATISE  ON  THE 
-L     DISEASES  OF  FEMALES.     Fourth  edition,  thoroughly  revised. 
In  one  large  and  handsome  octavo  volume  of  801  pages,  with  191 
illustrations.     Cloth,  $5  00  ;  leather,  $6  00.     (Just  issued.) 

TODD  (ROBERT  BENTLEY) .  CLINICAL  LECTURES  ON  CERTAIN 
ACUTE  DISEASES.  In  one  vol.  8vo.  of  320  pp.,  cloth,  $2  50. 

rpHOMPSON  (SIR  HENRY).    CLINICAL  LECTURES  ON  DISEASES 

-L  OF  THE  URINARY  ORGANS.  Second  and  revised  edition.  In 
one  8vo.  volume,  with  illustrations.  Cloth,  $2  25.  (Just  issued.) 

THE  PATHOLOGY  AND  TREATMENT  OF  STRICTURE  OF 

THE  URETHRA  AND  URINARY  FISTULA.  From  the  third 
English  edition.  In  one  8vo.  vol.  of  359  pp.,  with  illus.  Cloth,  $3  50. 

THE  DISEASES  OF  THE  PROSTATE,  THEIR  PATHOLOGY 

AND  TREATMENT.  Fourth  edition,  revised.  In  one  very  hand- 
some 8vo.  vol.  of  355  pp.,  with  13  plates.  Cloth,  $3  75. 


12  HENRY  C.  LEA'S  PUBLICATIONS. 

WALSHE  (W.  H.)  PRACTICAL  TREATISE  ON  THE  DISEASES 
OF  THE  HEART  AND  GREAT  VESSELS.  Third  American  from 
the  third  revised  London  edition.  In  one  8vo.  vol.  of  420  pages. 
Cloth,  $3. 

WATSON  (THOMAS).  LECTURES  ON  THE  PRINCIPLES  AND 
PRACTICE  OF  PHYSIC.  A  new  American  from  the  fifth  and  en- 
larged English  edition,  with  additions  by  H.  Hartshorne,  M,D.  In 
two  large  and  handsome  octavo  volumes.  Cloth,  $9  ;  leather,  $11. 

WOHLER'S  OUTLINES  OF  ORGANIC  CHEMISTRY.  Translated 
from  the  8th  German  edition,  by  Ira  Remsen,  M.D.  In  one  neat 
12mo.  vol.  Cloth,  $3  00.  (Lately  issued.') 

T/TTELLS    (J.  SOELBERG).    A   TREATISE   ON  THE  DISEASES  OF 

W   THE  EYE.     Second  American,  from  the  Third  English  edition,  with 

additions  by  I.  Minis  Hays,  M.D.     In  one  large  and  handsome  octavo 

vol.,  with  6  colored  plates  and  many  wood-cuts,  also  selections  from 

the  test-types  of  Jaeger  and  Snellen.     Cloth,  $5  00  ;  leather,  $6  00. 

TTITHAT  TO   OBSERVE  AT  THE   BEDSIDE  AND  AFTER  DEATH 

W  IN  MEDICAL  CASES.    In  one  royal  12mo.  vol.     Cloth,  $1. 

WEST  (CHARLES).  LECTURES  ON  THE  DISEASES  PECULIAR 
TO  WOMEN.  Third  American  from  the  Third  English  edition.  In 
one  octavo  volume  of  550  pages.  Cloth,  $3  75  ;  leather,  $4  75. 

LECTURES  ON  THE  DISEASES  OF  INFANCY  AND  CHILD- 
HOOD.    Fifth  American  from  the  sixth  revised  English  edition.     In 
one  large  8vo.  vol.  of  670  closely  printed  pages.     Cloth,  $4  50  ;  lea- 
ther, $5  50.     (Just  issued.) 

ON   SOME   DISORDERS    OF    THE   NERVOUS   SYSTEM   IN 

CHILDHOOD.     From  the  London   Edition.     In  one  small  12mo. 
volume.     Cloth,  $1. 

AN  ENQUIRY  INTO  THE  PATHOLOGICAL  IMPORTANCE 

OF  ULCERATION  OF  THE  OS  UTERI.     In  one  vol.  8™.     Cloth, 
$1  25. 

WILLIAMS  (CHARLES  J.  B.  and  C  T.)  PULMONARY  CONSUMP- 
TION :  ITS  NATURE,  VARIETIES,  AND  TREATMENT.  In 
one  neat  octavo  volume.  Cloth,  $2  50. 

TflTlLSON   (ERASMUS).     A  SYSTEM  OF  HUMAN  ANATOMY.     A 
•  *    new  and  revised  American  from  the  last  English  edition.    Illustrated 
with  397  engravings  on  wood.     In  one  handsome  8vo.  vol.  of  over 
600  pages.    Cloth,  $4  ;  leather,  $5. 

ON  DISEASES  OF  THE  SKIN.     The  seventh  American  from 

the  last  English  edition.     In  one  large  8vo.  vol.  of  over  800  pages 
Cloth,  $5. 

Also,  A  SERIES  OF  PLATES,  illustrating  "Wilson  on  Diseases  of  the 
Skin,"  consisting  of  20  plates,  thirteen  of  which  are  beautifully 
colored,  representing  about  one  hundred  varieties  of  Disease.  $5  50. 

Also,  the  TEXT  AND  PLATES,  bound  in  one  volume.     Cloth,  $10. 

THE  STUDENT'S  BOOK   OF  CUTANEOUS   MEDICINE.     In 

one  handsome  royal  12mo.  vol.     Cloth,  $3  50. 

TTC71NSLOW  (FORBES).    ON  OBSCURE  DISEASES  OF  THE  BRAIN 
W   AND  DISORDERS  OF  THE   MIND.     In  one  handsome  8vo.  vol. 
of  nearly  600  pages.    Cloth,  $4  25. 

WINCKEL  ON   PATHOLOGY  AND  TREATMENT  OF  CHILDBED. 
With  Additions  by'the  Author.     Translated  by  Chadwick.     In  one 
handsome  octavo  volume  of  484  pages.     Cloth,  $4.     (Just  issued.) 
I7EISSL    ON    VENEREAL    DISEASES.       Translated    by    Sturgis. 
"    (Preparing.) 


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